Trace Elements (Mg,Ca and Sr) Measurements in Marine Carbonates for Palaeothermometry: A Method Validation Using Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES)

Abstract

The magnesium-to-calcium (Mg/Ca) and strontium-to-calcium (Sr/Ca) ratios in certified reference materials (CRMs) and natural carbonate samples have been analysed using an inductively coupled plasma optical emission spectrometer (ICP-OES) for method validation. The CRMs included ECRM 752-1, a limestone standard and NIST SRM 915a, a pure calcium carbonate standard, while the marine carbonates included planktonic foraminifera (Globigerinoides ruber) from the southeastern Arabian Sea and large benthic foraminifera (Nummulites) from the Palaeogene Jaisalmer Basin, Rajasthan, India. The study achieved high precision and accurate analysis of Mg/Ca and Sr/Ca ratios. The observed values for ECRM 752-1 were consistent with certified data, exhibiting relative standard deviations (RSD) of 2.15% for Mg/Ca and 2.07% for Sr/Ca. Further, the SST reconstructions from Mg/Ca measurements of G. ruber indicated a temperature increase of approximately 4°C from the Last Glacial Maximum (LGM) to the Holocene. In contrast, the lower Mg/Ca and Sr/Ca ratios in Nummulites suggest post-depositional alteration, emphasising the necessity of considering diagenetic effects before deriving palaeotemperature, particularly for deep geological past samples. The reliability of ICP-OES in generating quality geochemical data for palaeothermometry is emphasised.

Keywords

Palaeothermometry biogenic carbonates ECRM 752-1 NIST SRM 915a trace elements ICP-OES

INTRODUCTION

Reconstructing past climatic and oceanographic conditions is important for understanding the Earth’s dynamic climate system, including its responses to natural and anthropogenic influences. The chemistry of marine carbonates, such as foraminiferal shells, corals and other biogenic deposits, acts as repositories of past environmental conditions (Anand et al., 2003; Dekens et al., 2002; Elderfield & Ganssen, 2000; Lea et al., 1999; Lear et al., 2002; Nuernberg, 1995; Rosenthal & Lohmann, 2002; Sadekov et al., 2016; Singh et al., 2022). The trace element ratios, particularly magnesium-to-calcium (Mg/Ca) and strontium-to-calcium (Sr/Ca), are used to estimate sea surface temperature (SST) (Greaves et al., 2008; Mortyn et al., 1997, 2005; Rosenthal & Lohmann, 2002), providing valuable insights into palaeoceanographic and palaeoclimatic variability across different geological timescales (Elderfield & Ganssen, 2000; Lea et al., 1999; Nuernberg, 1995). The Mg/Ca ratio in biogenic carbonates is primarily controlled by temperature, making it a sensitive proxy for SST reconstruction (de Villiers et al., 2002; Goodkin et al., 2007; Mitsuguchi et al., 2008; Rosenthal et al., 2004; Schöne et al., 2010; Yan et al., 2013). The incorporation of magnesium into calcite with an increase in temperature is empirically validated through laboratory experiments and sediment core studies (Barker et al., 2003; Rosenthal et al., 1997). The distribution coefficient (D) for Mg incorporation into calcite is proportionally related (D = (Mg/Ca)_calcite/(Mg/Ca)_seawater, where Mg and Ca are in molar proportions). However, several additional factors influence the Mg/Ca ratio, including salinity, pH, seawater Mg/Ca ratios and species-specific biological effects (Elderfield et al., 2002; Greaves et al., 2005; Rosenthal et al., 2004). In contrast, Sr/Ca ratios are less temperature-sensitive (relative to Mg/Ca) but provide complementary information, particularly in benthic environments and in the study of coral growth rates (Anand et al., 2003; Brown & Elderfield, 1996; Dekens et al., 2002; Elderfield & Ganssen, 2000; Hastings et al., 1998; Lea et al., 1999; Mashiotta et al., 1999; Nuernberg, 1996; Rosenthal & Lohmann, 2002; Rosenthal et al., 1997, 2000). While the Sr/Ca ratio in planktonic foraminifera shows a weaker response to temperature due to depth-dependent dissolution effects, yet it remains valuable for reconstructing palaeoceanographic processes (Elderfield et al., 1996; Mucci, 1987). Advancements in ICP based analytical techniques have enabled precise and accurate measurements of trace element ratios in marine carbonates. Though inductively coupled plasma-mass spectrometry (ICP-MS) offers superior sensitivity, ICP-OES remains widely used for palaeoclimate research because of its cost-effectiveness, high throughput, robustness, capability to measure multiple elements simultaneously and accessibility (Andreasen et al., 2006; Greaves et al., 2008; Rosenthal et al., 2004). However, interlaboratory discrepancies in Mg/Ca and Sr/Ca measurements necessitate the need for method validation and standardisation to ensure data comparability across studies (Greaves et al., 2005). The studies using foraminiferal shells and standard solutions indicate that within-laboratory precision (0.5%) is better than the interlaboratory precision (relative standard deviation (RSD) of 3.4% for Mg/Ca and 1.8% for Sr/Ca) (Rosenthal et al., 2004). These discrepancies arise from variations in cleaning protocols, instrumental conditions and calibration approaches (Barker et al., 2003; Mohtadi et al., 2014). Rigorous standardisation of analytical methods and calibration protocols is thus necessary to ensure reliable palaeotemperature estimates and their intercomparison. Therefore, the certified geological standard measurements are important in the standardisation and validation of trace element analyses. ECRM 752-1 (a limestone standard), with well-documented Mg/Ca and Sr/Ca values, is widely used in interlaboratory calibration studies, while NIST SRM 915a (a pure calcium carbonate standard) provides a high-purity matrix ideal for testing analytical methods for their detection limits (Greaves et al., 2005, 2008). The consistent use of such standards ensures the reliability of geochemical data and facilitates meaningful comparisons across different laboratories. This study aims to validate the analytical protocols for measuring Mg/Ca and Sr/Ca ratios using a newly installed ICP-OES at the Birbal Sahni Institute of Palaeosciences (BSIP), Lucknow, India. The validation process involved the analysis of certified geological reference materials (ECRM 752-1 and NIST SRM 915a) and focused on ensuring higher precision and accuracy for lower Mg/Ca ratios in biogenic carbonates. The sample preparation involved laborious cleaning protocols to remove contaminants that could bias the measurements of Mg/Ca and Sr/Ca. These protocols included oxidative and reductive cleaning steps to eliminate organic matter and adhere to established methodologies (Barker et al., 2003; Rosenthal et al., 1997). The geochemical data were validated through interlaboratory comparisons, error analyses and back-validation using estimated SST gradients from selected natural carbonate samples. The trace element ratios of biogenic carbonates, including Globigerina ruber samples from the southeastern Arabian Sea and Nummulites from the Jaisalmer Basin in western India, have been analysed to explore palaeothermometric conditions in Indian environments.

MATERIALS AND METHODS

Geological Reference Materials and Natural Samples

This study used two geological reference materials: ECRM 752-1 (limestone) and NIST SRM 915a (pure calcium carbonate). ECRM 752-1 is employed to validate the analytical methodology for measuring Mg/Ca and Sr/Ca ratios in marine carbonates. Additionally, Mg/Ca and Sr/Ca values are reported for NIST SRM 915a to establish benchmarks for lower concentrations, typical of certain foraminiferal species or foraminifera inhabiting low-temperature waters at higher latitudes. Carbonate saturation is very low at higher latitude (Mikis et al., 2019) and Mg incorporation in the foraminifera is temperature dependent; a slightly higher value of Mg can mislead the environmental conditions (Barret et al., 2025). A calibration standard having a similar range of Mg/Ca ratio is robust for cold water planktic foraminifera. ECRM 752-1, also referred to as BCS-CRM 393, is supplied in powdered form (<75 µm) by the Bureau of Analytical Samples Ltd., UK. This reference standard undergoes extensive homogeneity testing for Mg/Ca and Sr/Ca ratios through interlaboratory calibration studies, and its values are documented in the literature (Greaves et al., 2008). NIST SRM 915a, a pure calcium carbonate standard primarily used for atomic absorption and titrimetric analyses, is selected for its high purity and suitability for validating low-concentration measurements.

In addition to the CRMs, natural carbonate samples are analysed for Mg/Ca and Sr/Ca ratios. These include Globigerinoides ruber, a planktonic foraminiferal shell retrieved from sediment core SK-362_AE-2 in the southeastern Arabian Sea, and Nummulites shells collected from the Ter-e-Tekkar Member, Khuiala Formation, Palaeogene sequences of Jaisalmer Basin (Figure 1). The SK-362_AE-2 site is located at 12°9.78′N and 73°9.99′E, near the coast of Mangalore, with a water column depth of approximately 2,100 m. A 500 cm-long sediment core is retrieved from this location during the ORV Sagar-Kanya cruise (Cruise No. SK-362). The G. ruber samples are selected to validate SST variation since the Last Glacial Maximum (LGM) to the Holocene. The Nummulites shells, on the other hand, are analysed to investigate bottom water temperatures during the Palaeogene.

Figure 1.

Source: The image created using GeoMapApp (https://www.geomapapp.org/)

Multiple aliquots of these foraminiferal shells are independently prepared and analysed across different analytical batches to evaluate reproducibility and minimise bias. Rigorous sample preparation protocols are implemented, involving cleaning processes to eliminate potential contaminants and ensure reliable measurements. The results obtained from these natural samples and reference materials provide insights into palaeoclimatic and palaeoceanographic variability.

Measurement Standard and Sample Preparation Techniques

All measurements and sample preparation procedures are carried out in a clean laboratory space with HEPA-filtered workspace at the Geochemical Division, BSIP, Lucknow, India.

Preparation of Calibration Standards and Standard Reference Materials

The matrix-matching calibration standard ranges are prepared gravimetrically using mono-element standard solutions (1,000 mg/L; Inorganic Ventures/Merck) after appropriate dilutions. Fresh standards are prepared for each batch of measurements. Reference materials (ECRM 752-1 and NIST SRM 915a) are oven-dried at 100°C for 6 h to remove moisture prior to dissolution. Replicate aliquots of approximately 100–200 µg of ECRM 752-1 and NIST SRM 915a are taken in pre-cleaned polypropylene tubes. The aliquots are dissolved in 0.075M HNO₃ (Merck, Suprapur) approximately 12 h before measurement. Dissolution volumes are maintained in proportion to the Mg/Ca and Sr/Ca mixed calibration standard solutions to avoid matrix effects on emission intensity. In addition to geological reference materials, G. ruber shells with sizes of approximately 250–350 µm (~30 specimens at each depth) and single macroscopic specimens of Nummulites are also processed.

Carbonate Retrieval from Natural Samples

Approximately five samples of G. ruber (around 25–40 specimens at each stratigraphic level) and a single specimen of Nummulites from the Palaeogene sequences of the Jaisalmer Basin are used as a pilot study for palaeothermometry validation. For the collection of G. ruber, 5–7 g of sediment core samples is soaked overnight in deionised water. The sediment samples are then treated with hydrogen peroxide (3%, Merck, analytical grade) to remove organic matter. Clay particles are dispersed using sodium hexametaphosphate (15%, Merck, analytical grade) before sieving the sediments through a 63 µm nylon sieve. The sieved fraction is further dry-sieved to isolate foraminifera within the size range of 250–350 µm. Clean and well-preserved shells of G. ruber specimens are identified and separated under a microscope (METZ Stereozoom) for trace element analysis.

Nummulites samples are collected from the Ter-e-Tekkar Member of the Khuiala Formation, Jaisalmer Basin, Western Rajasthan, India. Chronologically, the Khuiala Formation has been dated to the early to mid-Eocene (Khanolkar et al., 2021; Singh, 2007). The Nummulites specimens are hand-picked from clastic sediments. Large benthic foraminifera (LBF) specimens (>2.5 mm radius) are soaked overnight in distilled water and treated with hydrogen peroxide to remove organic remains. The specimens are further processed similarly to G. ruber, as detailed above.

Sample Cleaning

Before analytical measurement, the foraminifera samples are processed following the cleaning procedure described in detail by Elderfield and Ganssen (2000), Barker et al. (2003) and Pang et al. (2020). A three-step cleaning protocol is followed, which includes: (a) the use of methanol for removing clay particles, (b) 1% H₂O₂ buffered with 0.1 M NaOH for removing organic matter and (c) 0.001 M HNO₃ solution to remove Fe and Mn oxyhydroxide coatings. Finally, the reference materials and foraminifera samples are dissolved in 0.075 M HNO₃ (0.1 M HNO₃ for dissolving Nummulites) before measurement using ICP-OES.

Analytical Techniques

The reference materials (ECRM 752-1 and NIST SRM 915a), LBF and G. ruber are subjected to trace element concentration measurements using ICP-OES (Agilent Technologies, 5800) at the Geochemistry Division of the BSIP, Lucknow. The instrumental conditions maintained during the measurements and the sample weights (mg) are summarised in Table 1. The sample solutions are aspirated into a cyclonic spray chamber through a micromist nebuliser using an autosampler (SPS-4, CETAC Technologies). Measurements are carried out in axial mode for trace elements (Mg, Sr, Fe, Mn, Al, etc.), while Ca is measured in radial mode due to its better sensitivity yield (Greaves et al., 2005). The measurement RSD remains below 1% for all samples and standards.

Table 1.

Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES) Instrumental Conditions During Elemental Concentration Measurements.

Category	Parameter	Setting
Plasma	Configuration	Radial (for Ca) and Axial
	RF power	1.2 kW
	Plasma gas flow	15 L/min
	Auxiliary gas flow	1.5 L/min
	Nebuliser gas flow	1.0 L/min
Nebuliser	Agilent, Sea Spray (glass) U-Series
Spray chamber	Agilent Double Pass	0.2 mL/min
Measurement	Integration time	5 s
Measurement	Replicates	3

The sensitivity dependence is checked before analysis by measuring ECRM 752-1 at different Ca concentrations. The concentrations of elements such as Ca, Mg, Sr, Al, Fe and Mn in carbonates are measured from multiple aliquots across different batches of instrumental analysis. During these measurements, multiple emission intensities are recorded simultaneously for each analyte. Stable emission wavelengths of 317.933 nm for Ca, 279.55 nm for Mg and 421.552 nm for Sr are selected in this study as their emission intensities yield better standard deviation (SD) and RSD (%). Measurements of blank solutions, calibration standards and reference materials (ECRM 752-1) are repeated after every five samples to monitor and correct for analytical drift in emission intensities.

The elemental concentrations of reference materials and natural samples are calculated from the emission intensities of calibration standards with known concentrations using a linear equation. The RSD% observed during the measurements of Ca, Mg, Sr, Fe and Mn is less than 1% for ECRM 752-1 across all batches, while it varies from 0.5% to 1.8% (for Mg) in NIST SRM 915a. The RSD for Ca, Mg and Sr remains below 1% for G. ruber and LBF samples during measurements. Negligible contamination from silicate matrices in the analysed samples is confirmed by low values of Fe/Mg (<0.04 mmol/mol), Fe/Ca (<0.15 mmol/mol), Mn/Ca (<0.4 mmol/mol) and undetectable Al/Ca, all of which are below the threshold value of 1 mmol/mol (Barker et al., 2003; Mohtadi et al., 2014). Furthermore, the poor correlation of the Mg/Ca ratio with Fe/Ca, Mn/Ca and Fe/Mg supports the efficiency of the cleaning protocol used during laboratory preparation (Anand et al., 2003).

Statistical Techniques

Outlier Identification. Measured data have undergone statistical treatment to identify outlier values using three sigma test, Z-score test and Cochran’s C test. Intra-batch SD has been considered as repeatability (Mg/Ca~0.042, and Sr/Ca~0.001 for ECRM, while Mg/Ca~0.007 and Sr/Ca~0.001 for NIST), while overall SD has been considered as reproducibility (Mg/Ca~0.097, and Sr/Ca~0.004 for ECRM, while Mg/Ca~0.014, and Sr/Ca~0.001 for NIST). Analysis of variance (ANOVA) was also performed.

3-sigma test (3 SD method): For each batch mean and SDs were calculated. Sample aliquots with values greater than (mean + 3*SD), or smaller than (mean – 3*SD) are considered outliers.

Z-score test: Z-score is calculated using formula $Z = \frac{|X_{i} - \underline{X}|}{σ}$ ; where $X_{i}$ is individual measurement, $\underline{X}$ is the batch mean and $σ$ is the batch SD. Sample aliquots having z-score > 2 or < –2 are considered outliers.

Cochran’s C test: It is a statistical technique to detect variance outliers among some groups of observations. In the first step, variance for each batch is calculated as $v a r = \frac{\sum {(X_{i} - \underline{X})}^{2}}{n - 1}$ ; where $v a r$ is sample variance, $X_{i}$ is individual measurement, $\underline{X}$ is the batch mean and $n$ is the number of sample aliquots in the batch. Then Cochran’s C test parameter is calculated as $C = \frac{(v a r)}{\sum (v a r)}$ ; where $(v a r)$ is the maximum variance and $\sum (v a r)$ is the sum of variances of all batches. The C-value thus obtained is compared with C_critical to identify batches with maximum variability.

After applying all these tests, no outliers were found, so the data are used for ANOVA.

SST Estimation and Statistical Techniques

The incorporation of Mg into calcareous shells depends on various factors, including water depth, salinity, pH, CO₂, morphology and temperature. However, temperature exerts a dominant control (Evans et al., 2016; Gray et al., 2018; Gray & Evans, 2019; Mathien-Blard & Bassinot, 2009). Despite these multiple factors influencing Mg incorporation into carbonate shells, this study focuses primarily on validating the Mg and Ca measurements in geological reference materials and carbonate shells. The equation used for estimating SST in G. ruber is validated through sediment trap studies conducted in various oceans (Anand et al., 2003).

The effect of water depth on SST is calibrated using the equation provided by Dekens et al. (2002), which suggests a negligible influence of depth on SST in the eastern Arabian Sea. SST (Equation 1) and the associated error (Equation 2) are estimated from the Mg/Ca molar ratio measured in G. ruber from the southeastern Arabian Sea using equations provided by Anand et al. (2003), Saraswat et al. (2013) and Mohtadi et al. (2014). The SST calibration and associated error calculation equations are presented below.

Mg/Ca = 0.38 \times \{e^{(0.090 \times T)}\}

(1)

where T represents SST in °C.

σ T^{2} = {(\partial T / \partial a \times σ a)}^{2} + {(\partial T / \partial b \times σ b)}^{2} + {\partial T / \partial (Mg/Ca) \times σ (Mg/Ca)}^{2}

(2)

where a = 0.090 °C ± 0.003 °C, b = 0.38 °C ± 0.02 °C,

∂T/∂a = –1/a² × [ln{(Mg/Ca)/b}], ∂T/∂b = –1/(a × b) and ∂T/∂(Mg/Ca) = 1/a × {1/(Mg/Ca)}

RESULTS

The data obtained for Mg/Ca and Sr/Ca molar ratios in reference materials and selected natural samples are summarised in this section. The mean measurement values, SD, RSD and 2-sigma error (2SE) for Mg/Ca and Sr/Ca ratios in geological reference materials measured during different analytical sessions are presented in Table 2. In ECRM, the Mg/Ca ratio ranges from 3.56 to 3.94 mmol/mol, with an average value of 3.76 mmol/mol (SD: 0.096, RSD: 2.56%, 2SE: 0.026; n = 53). The Sr/Ca ratio ranges from 0.165 to 0.184 mmol/mol, with an average value of 0.176 mmol/mol (SD: 0.004, RSD: 2.07%, 2SE: 0.001; n = 53). In NIST SRM 915a, the Mg/Ca ratio ranges from 0.105 to 0.167 mmol/mol, with an average value of 0.140 mmol/mol (SD: 0.014, RSD: 9.95%, 2SE: 0.004; n = 26), and the Sr/Ca ratio ranges from 0.225 to 0.229 mmol/mol, with an average value of 0.227 mmol/mol (SD: 0.001, RSD: 0.49%, 2SE: 0.0003; n = 26). The analytical precision of Mg/Ca and Sr/Ca measurements in the reference material (ECRM) improves significantly after centrifugation. In centrifuged aliquots of ECRM, the average Mg/Ca ratio is 3.80 mmol/mol (SD: 0.040, RSD: 1.08%, 2SE: 0.040; n = 4), and the Sr/Ca ratio is 0.180 mmol/mol (SD: 0.001, RSD: 0.29%, 2SE: 0.001; n = 4). As a result, all samples are centrifuged prior to analytical measurements.

Table 2.

Measured Elemental Ratios in Certified Reference Materials ECRM 752-1 and NIST SRM 915a Using Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES).

Date	Batch	Sample Aliquot	Weight	Mg/Ca	Sr/Ca	Mg/Ca				Sr/Ca
			(mg)	(mmol/mol)	(mmol/mol)	(mmol/mol)				(mmol/mol)
						Avg.	SD	RSD (%)	2SE	Avg.	SD	RSD (%)	2SE
(a) ECRM 752-1
31 December 2021	1	1	0.14	3.71	0.17	3.79	0.089	2.36	0.073	0.17	0.002	0.95	0.001
		2	0.15	3.89	0.18
		3	0.14	3.66	0.17
		4	0.27	3.85	0.18
		5	0.27	3.83	0.17
		6	0.29	3.80	0.18
1 January 2022	2	7	0.14	3.89	0.18	3.88	0.025	0.65	0.029	0.17	0.002	1.32	0.003
		8	0.22	3.90	0.18
		9	0.13	3.85	0.17
4 January 2022	3	10	0.17	3.65	0.17	3.68	0.025	0.69	0.025	0.17	0.001	0.80	0.001
		11	0.15	3.69	0.17
		12	0.21	3.70	0.17
		13	0.27	3.70	0.17
5 January 2022	4	14	0.16	3.90	0.17	3.92	0.018	0.46	0.018	0.17	0.001	0.61	0.001
		15	0.17	3.94	0.17
		16	0.13	3.92	0.17
		17	0.16	3.90	0.17
6 January 2022	5	18	0.15	3.75	0.18	3.73	0.029	0.79	0.024	0.18	0.001	0.75	0.001
		19	0.16	3.74	0.18
		20	0.16	3.77	0.18
		21	0.19	3.73	0.18
		22	0.18	3.70	0.18
		23	0.15	3.69	0.17
7 January 2022	6	24	0.15	3.78	0.18	3.80	0.015	0.38	0.012	0.18	0.001	0.78	0.001
		25	0.15	3.80	0.17
		26	0.15	3.82	0.18
		27	0.21	3.80	0.18
		28	0.14	3.77	0.18
		29	0.17	3.80	0.18
8 January 2022	7	30	0.16	3.71	0.18	3.75	0.034	0.90	0.027	0.18	0.000	0.27	0.000
		31	0.18	3.72	0.18
		32	0.18	3.77	0.18
		33	0.20	3.79	0.18
		34	0.15	3.77	0.18
		35	0.16	3.74	0.18
12 January 2022	8	36	0.12	3.67	0.17	3.66	0.022	0.59	0.018	0.18	0.001	0.67	0.001
		37	0.14	3.64	0.18
		38	0.14	3.65	0.18
		39	0.14	3.69	0.18
		40	0.13	3.63	0.18
		41	0.16	3.67	0.18
10 February 2022	9	42	2.76	3.89	0.18	3.68	0.123	3.35	0.087	0.18	0.003	1.54	0.002
		43	2.76	3.82	0.18
		46	2.76	3.57	0.18
		47	2.76	3.56	0.18
		48	2.76	3.59	0.18
		49	2.76	3.60	0.18
		50	2.76	3.70	0.18
		51	2.76	3.69	0.18
28 March 2022	10	52	2.47	3.78	0.18	3.80	0.040	1.08	0.040	0.18	0.001	0.29	0.001
		53	2.47	3.76	0.18
		54	2.47	3.86	0.18
		55	2.47	3.81	0.18
(b) NIST SRM 915a
31 December 2021	1	58	0.21	0.13	0.23	0.13	0.006	4.52	0.005	0.23	0.001	0.33	0.001
		59	0.17	0.13	0.23
		60	0.31	0.12	0.23
		61	0.29	0.13	0.23
		62	0.24	0.14	0.22
6 January 2022	2	63	0.50	0.14	0.23	0.14	0.012	7.99	0.010	0.23	0.001	0.46	0.001
		64	0.48	0.14	0.23
		65	0.43	0.15	0.23
		66	0.44	0.14	0.23
		67	0.44	0.16	0.23
7 January 2022	3	68	0.49	0.15	0.23	0.15	0.007	4.79	0.006	0.23	0.001	0.34	0.001
		69	0.41	0.16	0.23
		70	0.35	0.15	0.23
		71	0.35	0.15	0.23
		72	0.45	0.14	0.23
		73	0.36	0.14	0.23
8 January 2022	4	74	0.43	0.15	0.23	0.15	0.008	5.49	0.008	0.23	0.001	0.30	0.001
		75	0.46	0.15	0.23
		76	0.44	0.17	0.23
		77	0.38	0.15	0.23
12 January 2022	5	78	0.41	0.13	0.23	0.14	0.007	5.27	0.007	0.23	0.001	0.31	0.001
		79	0.51	0.14	0.23
		80	0.39	0.15	0.23
		81	0.46	0.14	0.23
25 January 2022	6	82	0.49	0.11	0.23	0.11	0.003	2.49	0.003	0.23	0.000	0.21	0.001
		83	0.45	0.11	0.23

Note: RSD: Relative standard deviation; SD: Standard deviation.

To the best of our knowledge, this study is the first to report Mg/Ca and Sr/Ca molar ratios for the pure calcium carbonate reference material NIST SRM 915a. The measurement of Mg/Ca ratios in NIST SRM 915a is more challenging compared to ECRM (RSD >10%) due to the low Mg concentration. However, optimal sensitivity is achieved by adjusting the sample weight and dilution factor within the calibration range. The obtained Mg/Ca and Sr/Ca molar ratios in G. ruber (southeastern Arabian Sea sediment) and Nummulites (Jaisalmer Basin) samples are provided in Table 3. The Mg/Ca ratio in G. ruber ranges from 3.69 to 5.56 mmol/mol, while the Sr/Ca ratio ranges from 1.42 to 1.50 mmol/mol. The Mg/Ca ratio in Nummulites ranges from 6.52 to 10.44 mmol/mol, with an average value of 7.50 mmol/mol, and the Sr/Ca ratio ranges from 0.45 to 0.66 mmol/mol, with an average value of 0.48 mmol/mol. The overall SD of Mg/Ca from replicate measurements (n = 3) of G. ruber samples (n = 5) ranges from 0.03 to 0.16, while replicate measurements (n = 3) of Nummulites samples (n = 4) show an SD ranging from 0.05 to 0.33. Here, n represents the number of samples.

Table 3.

Measurement of Elemental Ratios (Mg/Ca and Sr/Ca Molar Ratios), Mg/Ca Molar Ratio Based Sea Surface Temperature (SST) and Its Standard Deviation (SD) in Planktonic (Globigerinoides ruber) and Large Benthic Foraminifera (LBF).

Species	Sl. No.	Depth	Age	Mg/Ca	Sr/Ca	Fe/Ca	Mn/Ca	Fe/Mg	SST	SD (SST)
Species	Sl. No.	(cm)	(Ka)	(mmol/mol)	(mmol/mol)	(mmol/mol)	(mmol/mol)	(mmol/mol)	°C	°C
Globigerinoides ruber	1	1	2.04	4.76	1.47	0.13	0.24	0.03	28.20	1.12
	2	1	2.04	4.86	1.48	0.08	0.24	0.02	28.20	1.12
	3	5	6.54	5.47	1.49	0.10	0.15	0.02	29.73	1.16
	4	5	6.54	5.56	1.51	0.10	0.16	0.02	29.73	1.16
	5	35	10.54	4.86	1.48	0.11	0.20	0.02	28.28	1.11
	6	35	10.54	4.82	1.50	0.09	0.22	0.02	28.28	1.11
	7	238	19.82	3.65	1.42	0.15	0.44	0.04	25.32	1.05
	8			3.80	1.43	0.14	0.43	0.04
	9			3.69	1.42	0.13	0.43	0.04
	10	246	20.17	3.95	1.44	0.11	0.28	0.03	25.68	1.14
	11	246	20.17	3.72	1.43	0.11	0.30	0.03	25.68	1.14
LBF	12		Early to mid-Eocene	7.47	0.45	0.20	0.17	0.03
	13			7.97	0.46	0.18	0.16	0.02
	14			7.95	0.45	0.15	0.16	0.02
	15			7.90	0.52	0.14	0.18	0.02
	16			7.68	0.50	0.20	0.13	0.03
	17			6.99	0.49	0.16	0.15	0.02
	18			6.52	0.49	0.13	0.11	0.02
	20			10.44	0.63	0.07	0.08	0.01
	21			10.37	0.66	0.08	0.12	0.01

DISCUSSION

A detailed discussion on the multiple measurements of standard reference materials and natural samples carried out using ICP-OES at BSIP is provided in this section to demonstrate the accurate and precise generation of a database for Mg/Ca and Sr/Ca molar ratios and their applicability for palaeothermometry.

Measurements of Mg/Ca and Sr/Ca in Standard Reference Materials, ECRM 752-1 and NIST SRM 915a

The mean value, SD, RSD and 2-sigma error (2SE) for each batch are calculated to evaluate the systematic and random errors associated with the measurements of ECRM 752-1 and NIST SRM 915a using ICP-OES at BSIP (Table 2). Systematic error is generally associated with the application of different cleaning protocols and contamination from reagents during sample preparation and measurements (Rosenthal et al., 2004). These systematic errors are addressed by applying blank and standard corrections for each aliquot of both reference materials and natural samples. The measurement SD and RSD for each batch are compared to evaluate the uncertainty associated with analytical measurements. The mean values and SD of Mg/Ca and Sr/Ca molar ratios are plotted against the date of measurements for the two reference materials (Figure 1). The Mg/Ca molar ratio of ECRM for each batch has an SD of less than 0.033 mmol/mol, except for batch 1 (0.089 mmol/mol) and batch 9 (0.123 mmol/mol). The lower SD and RSD in most batch measurements suggest negligible contamination or minimal measurement error. For batch 1, the SD and RSD are 0.089 mmol/mol and 2.36%, respectively, while for batch 9, they are 0.123 mmol/mol and 3.35%, respectively, indicating relatively higher uncertainty in these measurement runs compared to the other batches. However, the Mg/Ca molar ratios for batch 1 and batch 9 range from 3.66 to 3.89 mmol/mol and 3.56 to 3.89 mmol/mol, respectively, demonstrating much lower SD and RSD than the reported values for ECRM (Rosenthal et al., 2004). The scope for error is negligible in the Sr/Ca molar ratio of ECRM (Figure 2) as the SD and RSD vary between 0.004 and 0.002 mmol/mol and 0.27%–1.54%, respectively, which are better than the interlaboratory comparison values (Greaves et al., 2008; Rosenthal et al., 2004).

Figure 2.

Plots show date of measurements vs. elemental ratios in two certified reference materials, ECRM 752-1 (Mg/Ca in (A) and Sr/Ca in (B)) and NIST 915a(Mg/Ca in (C) and Sr/Ca in (D)).

For NIST SRM 915a, the SD and RSD of the Mg/Ca molar ratio for each batch are less than 0.001 mmol/mol and approximately 5%, respectively, except for batch 2, where they are 0.01 mmol/mol and 7.99%, respectively. The slightly higher RSD for batch 2 is attributed to the poor repeatability of replicate aliquots, likely due to random error. The SD (0.001 mmol/mol) and RSD (<0.40%) of the Sr/Ca molar ratio remain consistent across all batch measurements. Measuring Mg/Ca in NIST SRM 915a is more challenging due to the low Mg concentration. The certificate of analysis (CoA) for NIST SRM 915a indicates that smaller sample sizes (<250 mg) may lead to inhomogeneity (https://www-s.nist.gov/srmors/certificates/archive/915a.pdf). Despite this, better RSD for Sr/Ca and relatively poorer RSD for Mg/Ca in NIST SRM 915a suggest that sample inhomogeneity has a negligible effect on these measurements. The precise and accurate measurements of Mg/Ca in NIST SRM 915a (low concentration range) indicate that the analytical method demonstrated with ICP-OES at BSIP achieves better detection limits compared to previously reported values (Rodríguez & Not, 2021). It is also observed that the Sr/Ca molar ratio exhibits relatively better precision and repeatability compared to Mg/Ca molar ratio measurements for both ECRM 752-1 and NIST SRM 915a.

Measurements of Mg/Ca and Sr/Ca in Natural Carbonate Samples

The preliminary palaeothermometry study is conducted using planktonic and benthic foraminifera from the late Quaternary (collected from the southeastern Arabian Sea) and the Palaeogene (collected from the Jaisalmer Basin, Rajasthan), respectively, to validate the measurement protocols of Mg/Ca and Sr/Ca molar ratios for critical climatic intervals in and around the tropical Indian Ocean region during the LGM and the Holocene. However, this study does not focus on past SST conditions or palaeoceanographic processes in the southeastern Arabian Sea or the deep-time palaeothermometry of the Jaisalmer Basin, which will be discussed in detail in follow-up publications. The new measurements of Mg/Ca molar ratios in these samples provide robust temperature constraints for climatic events of the late Quaternary, while the selected LBF samples from the Palaeogene reflect post-depositional alteration, preventing temperature estimation.

Mg/Ca and Sr/Ca in Planktonic Foraminifera

The average values of Mg/Ca and Sr/Ca in G. ruber from the southeastern Arabian Sea are plotted in Figure 3, along with the estimated SST based on the Mg/Ca ratio. The elemental measurements are validated through error analysis in SST estimation propagated from the application of the Mg/Ca molar ratio. As previous studies suggest no clear dependence of Sr/Ca on calcification temperature in G. ruber (Cléroux et al., 2008), only the Mg/Ca molar ratio is used to calculate SST in this study. The SST estimated for the southeastern Arabian Sea during the LGM and late Holocene periods ranges from 25.3°C to 29.7°C. SST increases by approximately 4°C from the LGM to the late Holocene for the tropical Indian Ocean (Singh et al., 2022). Therefore, samples from the LGM and Holocene periods are selected to compare the results with published literature records. The lower temperatures during the LGM and higher temperatures during the late Holocene are consistent with hydrological changes in the tropical Indian Ocean region driven by North Atlantic climate forcing during the late Quaternary (Mohtadi et al., 2014). The Lakshadweep Sea sediments revealed SST variations exceeding 31°C during May (mini warm pool) and a 4°C decrease in SST during August–September (Shankar et al., 2004). The Mg/Ca-based SST gradient varied from 3°C to 4°C in the northern and eastern Arabian Sea, respectively, during the LGM to the Holocene (Anand et al., 2008; Banakar et al., 2010; Gaye et al., 2018; Govil & Naidu, 2010; Mahesh & Banakar, 2014; Saraswat et al., 2013; Singh et al., 2022). Furthermore, the temperature values obtained in this study for the southeastern Arabian Sea, based on Mg/Ca measurements of late Quaternary carbonates, align with previously reported values (Singh et al., 2022; Figure 4). The error associated with Mg/Ca measurements for SST reconstruction shows an SD of approximately 1°C for the southeastern Arabian Sea, which falls within acceptable limits (Mohtadi et al., 2014). The estimated SST and its associated error, propagated from Mg/Ca molar ratio measurements in G. ruber from the southeastern Arabian Sea, confirm the precision of trace element measurements in carbonates. The low matrix concentrations (Fe/Ca, Mn/Ca and Fe/Mg) observed in the measured G. ruber samples coincide with previous studies, supporting the accuracy and precision of the cleaning protocol (Figure 3). Thus, it is validated that the analytical measurements of Mg/Ca using ICP-OES at BSIP are highly accurate and precise for palaeothermometry reconstruction.

Figure 3.

Plot shows Mg/Ca molar ratio vs. Mg/Ca molar ratio-based SST and Sr/Ca molar ratio in planktonic foraminifera (G. ruber) of eastern Arabian Sea (SK-362_AE2). The horizontal error bar indicates the standard deviation of Mg/Ca while the vertical error bars indicate standard deviation of SST (primary axis) and Sr/Ca (secondary axis).

Figure 4.

Plot shows sea surface temperature (SST) estimated using the Mg/Ca molar ratio in planktonic foraminifera (G. ruber) of southeastern Arabian Sea (SK-362_AE2) and its comparison with the SST data from the eastern Arabian Sea (Singh et al., 2022; Govil and Naidu, 2010; Mahesh and Banakar, 2014 and Mohtadi et al., 2014).

Mg/Ca and Sr/Ca in Benthic Foraminifera and Calcification Temperature During Palaeogene

Nummulitids, LBF belonging to the Rotaliid family, are considered robust proxies for palaeothermometry due to their lower Mg-fractionation compared to planktonic and deep-sea benthic foraminifera (Beavington-Penney & Racey, 2004; Evans et al., 2015). The Mg/Ca ratio in LBF primarily depends on the ambient seawater temperature in Eocene sediments (Evans et al., 2013, 2015; Khanolkar et al., 2021). However, a systematic study on Nummulites explores the relationship between Mg/Ca concentrations, test size, preservation state and the application of various cleaning techniques (Martens et al., 2022). This study indicates that taphonomic alteration significantly affects Mg/Ca and Sr/Ca ratios in Nummulites compared to shell size and cleaning techniques, although these factors also contribute to some extent to the variation in elemental ratios (Evans et al., 2015). Well-preserved specimens and geochemistry of smaller-sized Nummulites (<3 mm) are robust for reconstructing Eocene palaeothermometry.

The average Mg/Ca and Sr/Ca values of seven replicate analyses of LBF specimens (<3 mm) are 7.497 mmol/mol and 0.479 mmol/mol, respectively. The SD and RSD of Mg/Ca for these seven aliquots are 0.51 mmol/mol and 6.82%, while for Sr/Ca, they are 0.02 mmol/mol and 5.20%, respectively (Table 3). Similarly, the SD and RSD of Mg/Ca for two additional aliquots are 0.03 mmol/mol and 0.33%, while for Sr/Ca, they are 0.01 mmol/mol and 2.08%. The consistent Mg/Ca and Sr/Ca ratios measured in Nummulites samples indicate precise and accurate analyses (Table 3).

The obtained elemental ratios in Nummulites from the Jaisalmer Basin are significantly lower than previously reported data for the Palaeogene (Martens et al., 2022). Martens et al. (2022) suggest that Eocene Nummulites typically exhibit higher Mg concentrations, and the lower Mg observed in this study may result from post-depositional alteration. The low Mg/Ca (<50 mmol/mol) and Sr/Ca (<1.5 mmol/mol) ratios in Nummulites are likely due to diagenetic Mg loss during taphonomic alteration rather than analytical error (Martens et al., 2022). Furthermore, the lower Mg/Ca (~7–10 mmol/mol) and Sr/Ca (0.45–0.66 mmol/mol) ratios observed in this study are consistent with previous findings, suggesting similar taphonomic alterations in the shell chemistry of Nummulites from the Jaisalmer Basin (Figure 5).

Figure 5.

Plot shows Sr/Ca vs Mg/Ca molar ratio in planktonic benthic foraminifera (Nummulites) from Jaisalmer Basin. The studied shells are taphonomically alterated.

Matrix element concentrations in LBF, including Fe/Ca, Mn/Ca and Fe/Mg, range from 0.129–0.204 mmol/mol, 0.114–0.179 mmol/mol and 0.017–0.027 mmol/mol, respectively (Figure 5). These matrix element values in Nummulites from the Jaisalmer Basin are better than those previously reported by Martens et al. (2022), further supporting the accuracy and precision of ICP-OES measurements at BSIP. The selected Nummulites samples in this study appear to be post-depositionally altered and are used to confirm the low Mg/Ca and Sr/Ca values in taphonomically altered carbonates rather than for SST estimation. Therefore, the Mg/Ca ratio in unaltered LBF from this basin is deemed suitable for reconstructing Eocene palaeothermometry. The lower Mg/Ca and Sr/Ca ratios observed in altered LBF from the Palaeogene in this study validate the analytical accuracy of elemental ratios obtained using ICP-OES at BSIP.

CONCLUSIONS

This study demonstrated that accurate and precise geochemical data for Mg/Ca and Sr/Ca molar ratios in carbonates can be reliably obtained using the ICP-OES facility at BSIP, Lucknow. The SD and RSD values for the measurement of these elemental ratios in ECRM 752-1 and NIST SRM 915a were comparable to certified values and interlaboratory-calibrated values for both Mg/Ca and Sr/Ca. Minor deviations in RSD across analytical batches, attributed to standard uncertainties associated with measurements, consistently remain within acceptable 2-sigma error limits, ensuring robust and dependable data quality. Further, the application of Mg/Ca for SST reconstruction using Globigerina ruber samples from the southeastern Arabian Sea and Nummulites from the Jaisalmer Basin was carried out. The observed SST increase of approximately 4°C from the LGM to the Holocene in the southeastern Arabian Sea, with an associated error margin (SD) of about 1°C, validates the reliability of Mg/Ca analyses in late Quaternary natural carbonates. Additionally, Mg/Ca and Sr/Ca molar ratios in Nummulites from the Palaeogene sediments of the Jaisalmer Basin, Rajasthan, are lower than averages reported for the Palaeogene sequences, suggesting the possible diagenetic loss of Mg during limestone preservation rather than inaccuracies in the analytical protocol. Thus, the sensitivity and precision of the ICP-OES system in detecting subtle geochemical variations in altered and unaltered carbonates were validated with certified standards and natural samples. This validated methodology provides a critical tool for unravelling past environmental and climatic conditions.

Footnotes

Acknowledgements

The Director, BSIP, Lucknow, and Director, NIH, Roorkee, are thanked for support and encouragement at various stages of this work (RDCC/2021-22/L-01 and NIH/HID/R&D/2025-26/05). The authors thank Dr Poornachandra Rao, Former Director, NCESS; Dr TN Prakash, Scientist (Rtd.), NCESS; Dr Anil Kumar, Scientist (Rtd.), NCPOR; Dr Sheela Nair, Scientist & Head, Marine Geoscience Group, NCESS and Ministry of Earth Sciences (MoES), Government of India, for the SK-362 expedition participation opportunity and support to TM and GPG. The Chief Scientist of the SK-362 expedition and ship crew members of Sagar Kanya are thanked for their support during sample collection. The authors also thank the Inter-University Accelerator Centre (IUAC), Delhi, for the radiocarbon dating of samples.

Declaration of Conflicting Interests

The authors declare no potential conflicts of interest regarding the research, authorship and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship and/or publication of this article: The Department of Science and Technology (DST), Government of India, is thanked for funding the laboratory set-up for inductively coupled plasma optical emission spectroscopy (ICP-OES) in BSIP as a part of Sophisticated Analytical Instrumentation Facility (SAIF).

References

Anand

, Elderfield

, & Conte

M. H.

(2003). Calibration of Mg/Ca thermometry in planktonic foraminifera from a sediment trap time series. Paleoceanography , 18(2). https://doi.org/10.1029/2002PA000846

Anand

, Kroon

, Singh

A. D.

, Ganeshram

R. S.

, Ganssen

, & Elderfield

(2008). Coupled sea surface temperature–seawater δ18O reconstructions in the Arabian Sea at the millennial scale for the last 35 ka. Paleoceanography , 23(4).

Andreasen

D. H.

, Sosdian

, Perron-Cashman

, Lear

C. H.

, de Garidel-Thoron

, Field

, & Rosenthal

(2006). Fidelity of radially viewed ICP-OES and magnetic-sector ICP-MS measurement of Mg/Ca and Sr/Ca ratios in marine biogenic carbonates: Are they trustworthy together? Geochemistry Geophysics , 7(10). https://doi.org/10.1029/2005GC001124

Banakar

V. K.

, Mahesh

B. S.

, Burr

, & Chodankar

A. R.

(2010). Climatology of the Eastern Arabian Sea during the last glacial cycle reconstructed from paired measurement of foraminiferal δ18O and Mg/Ca. Quaternary Research , 73(3), 535–540.

Barker

, Greaves

, & Elderfield

(2003). A study of cleaning procedures used for foraminiferal Mg/Ca paleothermometry. Geochemistry Geophysics , 4(9). https://doi.org/10.1029/2003GC000559

Barrett

, de Vries

, & Schmidt

D. N.

(2025). What controls planktic foraminiferal calcification? Biogeosciences , 22(3), 791–807.

Beavington-Penney

S. J.

, & Racey

(2004). Ecology of extant nummulitids and other larger benthic foraminifera: Applications in palaeoenvironmental analysis. Earth-Science Reviews , 67(3–4), 219–265.

Brown

S. J.

, & Elderfield

(1996). Variations in Mg/Ca and Sr/Ca ratios of planktonic foraminifera caused by postdepositional dissolution: Evidence of shallow Mg-dependent dissolution. Paleoceanography , 11(5), 543–551.

Cléroux

, Cortijo

, Anand

, Labeyrie

, Bassinot

, Caillon

, & Duplessy

J. C.

(2008). Mg/Ca and Sr/Ca ratios in planktonic foraminifera: Proxies for upper water column temperature reconstruction. Paleoceanography , 23(3). https://doi.org/10.1029/2007PA001505

10.

de Villiers

, Greaves

, & Elderfield

(2002). An intensity ratio calibration method for the accurate determination of Mg/Ca and Sr/Ca of marine carbonates by ICP-AES. Geochemistry Geophysics , 3(1). https://doi.org/10.1029/2001GC000169

11.

Dekens

P. S.

, Lea

D. W.

, Pak

D. K.

, & Spero

H. J.

(2002). Core top calibration of Mg/Ca in tropical foraminifera: Refining paleotemperature estimation. Geochemistry Geophysics , 3(4), 1–29. https://doi.org/10.1029/2001GC000200

12.

Elderfield

, Bertram

C. J.

, & Erez

(1996). A biomineralization model for the incorporation of trace elements into foraminiferal calcium carbonate. Earth & Planetary Science Letters , 142, 409–423. https://doi.org/10.1016/0012-821X(96)00105-7

13.

Elderfield

, & Ganssen

(2000). Past temperature and 18O of surface ocean waters inferred from foraminifera Mg/Ca ratios. Nature , 405, 442–445. https://doi.org/10.1038/35013033

14.

Elderfield

, Vautravers

, & Cooper

(2002). The relationship between shell size and Mg/Ca, Sr/Ca, δ18O, and δ13C of species of planktonic foraminifera. Geochemistry, Geophysics, Geosystems , 3(8), 1–13.

15.

Evans

, Müller

, Oron

, & Renema

(2013). Eocene seasonality and seawater alkaline earth reconstruction using shallow-dwelling large benthic foraminifera. Earth and Planetary Science Letters , 381, 104–115.

16.

Evans

, Wade

B. S.

, Henehan

, Erez

, & Muller

(2015). Revisiting carbonate chemistry controls on planktic foraminifera Mg/Ca: Implications for sea surface temperature and hydrology shifts over the Paleocene–Eocene Thermal Maximum and Eocene–Oligocene Transition. Climate of the Past , 11, 3143–3185.

17.

Evans

, Brierley

, Raymo

M. E.

, Erez

, & Müller

(2016). Planktic foraminifera shell chemistry response to seawater chemistry: Pliocene–Pleistocene seawater Mg/Ca, temperature and sea level change. Earth and Planetary Science Letters , 438, 139–148.

18.

Gaye

, Böll

, Segschneider

, Burdanowitz

, Emeis

K. C.

, Ramaswamy

, Lahajnar

, Lückge

, & Rixen

(2018). Glacial–interglacial changes and Holocene variations in Arabian Sea denitrification. Biogeosciences , 15(2), 507–527.

19.

Goodkin

N. F.

, Hughen

K. A.

, & Cohen

A. L.

(2007). A multicoral calibration method to approximate a universal equation relating Sr/Ca and growth rate to sea surface temperature. Paleoceanography , 22(1), 12–14. https://doi.org/10.1029/2006PA001312

20.

Govil

, & Naidu

P. D.

(2010). Evaporation-precipitation changes in the eastern Arabian Sea for the last 68 ka: Implications on monsoon variability. Paleoceanography , 25(1). https://doi.org/10.1029/2008PA001687

21.

Gray

W. R.

, & Evans

(2019). Nonthermal influences on Mg/Ca in planktonic foraminifera: A review of culture studies and application to the Last Glacial Maximum. Paleoceanography and Paleoclimatology , 34(3), 306–315.

22.

Gray

W. R.

, Weldeab

, Lea

D. W.

, Rosenthal

, Gruber

, Donner

, & Fischer

(2018). The effects of temperature, salinity, and the carbonate system on Mg/Ca in Globigerinoides ruber (white): A global sediment trap calibration. Earth and Planetary Science Letters , 482, 607–620.

23.

Greaves

, Barker

, Daunt

, & Elderfield

(2005). Accuracy, standardization, and interlaboratory calibration standards for foraminiferal Mg/Ca thermometry. Geochemistry Geophysics , 6(2). https://doi.org/10.1029/2004GC000790

24.

Greaves

, Caillon

, Rebaubier

, Bartoli

, Bohaty

, Cacho

, Clarke

, Cooper

, Daunt

, Delaney

, de Menocal

, Dutton

, Eggins

, Elderfield

, Garbe-Schoenberg

, Goddard

, Green

, Groeneveld

, Hasting

, … Wilson

P. A.

(2008). Interlaboratory comparison study of calibration standards for foraminiferal Mg/Ca thermometry. Geochemistry Geophysics , 9(8). https://doi.org/10.1029/2008GC001974

25.

Hastings

D. W.

, Russell

A. D.

, & Emerson

S. R.

(1998). Foraminiferal magnesium in Globigerinoides sacculifer as a paleotemperature proxy. Paleoceanography , 13(2), 161–169.

26.

Khanolkar

, Choudhury

T. R.

, Saraswati

P. K.

, & Banerjee

(2021). Late Paleocene-early Eocene foraminiferal assemblage and carbon isotope excursion indicating hyperthermal events in paleotropical succession of northwestern India. Journal of Foraminiferal Research , 51(1), 4–13. https://doi.org/10.2113/gsjfr.52.1.40

27.

Lea

D. W.

, Mashiotta

T. A.

, & Spero

H. J.

(1999). Controls on magnesium and strontium uptake in planktonic foraminifera determined by live culturing. Geochimica et Cosmochimica Acta , 63(16), 2369–2379. https://doi.org/10.1016/S0016-7037(99)00197-0

28.

Lear

C. H.

, Rosenthal

, & Slowey

(2002). Benthic foraminiferal Mg/Ca-paleothermometry: A revised core-top calibration. Geochimica et Cosmochimica Acta , 66(19), 3375–3387. https://doi.org/10.1016/S0016-7037(02)00941-9

29.

Mahesh

B. S.

, & Banakar

V. K.

(2014). Change in the intensity of low-salinity water inflow from the Bay of Bengal into the Eastern Arabian Sea from the Last Glacial Maximum to the Holocene: Implications for monsoon variations. Palaeogeography, Palaeoclimatology, Palaeoecology , 397, 31–37.

30.

Martens

, Stassen

, Steurbaut

, & Speijer

R. P.

(2022). Assessing Nummulites geochemistry as a proxy for early Eocene palaeotemperature evolution in the North Sea Basin. Journal of Geological Society . https://doi.org/10.1144/jgs2021-102

31.

Mashiotta

T. A.

, Lea

D. W.

, & Spero

H. J.

(1999). Glacial–interglacial changes in Subantarctic sea surface temperature and δ18O-water using foraminiferal Mg. Earth and Planetary Science Letters , 170(4), 417–432.

32.

Mathien-Blard

, & Bassinot

(2009). Salinity bias on the foraminifera Mg/Ca thermometry: Correction procedure and implications for past ocean hydrographic reconstructions. Geochemistry, Geophysics, Geosystems , 10(12).

33.

Mikis

, Hendry

K. R.

, Pike

, Schmidt

D. N.

, Edgar

K. M.

, Peck

, Peeters

F. J. C.

, Leng

M. J.

, Meredith

M. P.

, Jones

C. L. C.

, Stammerjohn

, & Ducklow

(2019). Temporal variability in foraminiferal morphology and geochemistry at the West Antarctic Peninsula: a sediment trap study. Biogeosciences , 16(16), 3267–3282.

34.

Mitsuguchi

, Dang

P. X.

, Kitagawa

, Uchida

, & Shibata

(2008). Coral Sr/Ca and Mg/Ca records in Con Dao Island off the Mekong Delta: Assessment of their potential for monitoring ENSO and East Asian monsoon. Global Planetary Change , 63(4), 341–352. https://doi.org/10.1016/j.gloplacha.2008.08.002

35.

Mohtadi

, Prange

, Oppo

D. W.

, Pol-Holz

R. D.

, Merkel

, Zhang

, Steinke

, & Lückge

(2014). North Atlantic forcing of tropical Indian Ocean climate. Nature , 509(7), 6–80. https://doi.org/10.1038/nature13196

36.

Mortyn

P. G.

, Thunell

R. C.

, & Anderson

D. M.

(1997). Sea surface temperature changes in the southern California Borderlands during the last glacial-in-terglacial cycle. Oceanographic Literature Review , 3(44), 215.

37.

Mortyn

P. G.

, Elderfield

, Anand

, & Greaves

(2005). An evaluation of controls on planktonic foraminiferal Sr/Ca: Comparison of water column and core-top data from a North Atlantic transect. Geochemistry, Geophysics, Geosystems , 6(12).

38.

Mucci

(1987). Influence of temperature on the composition of magnesian calcite overgrowths precipitated from seawater. Geochimica et Cosmochimica Acta , 51(7), 1977–1984. https://doi.org/10.1016/0016-7037(87)90186-4

39.

Nuernberg

(1995). Magnesium in tests of Neogloboquadrina pachyderma sinistral from high northern and southern latitudes. Journal of Foraminiferal Research , 25(4), 350–368. https://doi.org/10.2113/gsjfr.25.4.350

40.

Pang

, Bassinot

, & Sepulcre

(2020). Cleaning method impact on the Mg/Ca of three planktonic foraminifer species: A downcore study along a depth transect. Chemical Geology , 549, 119690.

41.

Rodríguez

, & Not

(2021). Calibration of Mg/Ca and Sr/Ca in coastal marine ostracods as a proxy for temperature. Biogeosciences , 18(6), 1987–2001. https://doi.org/10.5194/bg-18-1987-2021

42.

Rosenthal

, Boyle

E. A.

, & Slowey

(1997). Temperature control on the incorporation of magnesium, strontium, fluorine, and cadmium into benthic foraminiferal shells from Little Bahama Bank: Prospects for thermocline paleoceanography. Geochimica et Cosmochimica Acta , 61(17), 3633–3643.

43.

Rosenthal

, & Lohmann

G. P.

(2002). Accurate estimation of sea surface temperatures using dissolution-corrected calibrations for Mg/Ca paleothermometry. Paleoceanography , 17(3), 16-1–16–6. https://doi.org/10.1029/2001PA000749

44.

Rosenthal

, Lohmann

G. P.

, Lohmann

K. C.

, & Sherrell

R. M.

(2000). Incorporation and preservation of Mg in Globigerinoides sacculifer: Implications for reconstructing the temperature and 18O/16O of seawater. Paleoceanography , 15(1), 135–145.

45.

Rosenthal

, Perron-Cashman

, Lear

, Bard

, Barker

, Billups

, Bryan

, Delaney

M. L.

, de Menocal

P. B.

, Dwyer

G. S.

, Elderfield

, German

C. R.

, Greaves

, Lea

D. W.

, Marchitto

, Pak

D. K.

, Paradis

G. L.

, Russel

A. D.

, Schneider

R. R.

, … Wilson

P. A.

(2004). Interlaboratory comparison study of Mg/Ca and Sr/Ca measurements in planktonic foraminifera for paleoceanographic research. Geochemistry Geophysics , 5(4). https://doi.org/10.1029/2003GC000650

46.

Ryan

W.B.F.

, Carbotte

S.M.

, Coplan

, O’Hara

, Melkonian

, Arko

, Weissel

R.A.

, Ferrini

, Goodwillie

, Nitsche

, Bonczkowski

and Zemsky

(2009) Global multi-resolution topography (GMRT) synthesis data set. Geochemistry Geophysics Geosystems , 10. https://doi.org/10.1029/2008GC002332

47.

Sadekov

A. Y.

, Darling

K. F.

, Ishimura

, Wade

C. M.

, Kimoto

, Singh

A. D.

, Anand

, Kroon

, Jung

, Gansse

, & Ganeshram

(2016). Geochemical imprints of genotypic variants of Globigerina bulloides in the Arabian Sea. Paleoceanography , 31(10), 1440–1452. https://doi.org/10.1002/2016PA002947

48.

Saraswat

, Lea

D. W.

, Nigam

, Mackensen

, & Naik

D. K.

(2013). Deglaciation in the tropical Indian Ocean driven by interplay between the regional monsoon and global teleconnections. Earth & Planetary Science Letters , 375, 166–175. https://doi.org/10.1016/j.epsl.2013.05.022

49.

Schöne

B. R.

, Zhang

, Jacob

, Gillikin

D. P.

, Tütken

, Garbe-Schönberg

, McConnaughey

, & Soldati

(2010). Effect of organic matrices on the determination of the trace element chemistry (Mg, Sr, Mg/Ca, Sr/Ca) of aragonitic bivalve shells (Arctica islandica)—Comparison of ICP-OES and LA-ICP-MS data. Geochemical Journal , 44(1), 23–37.

50.

Shankar

, Gopalakrishna

V. V.

, Shenoi

S. S. C.

, Durand

, Shetye

S. R.

, Rajan

C. K.

, Johnson

, Araligidad

, & Michael

G. S.

(2004). Observational evidence for westward propagation of temperature inversions in the southeastern Arabian Sea. Geophysical Research Letters , 31(8).

51.

Singh

N. P.

(2007). Cenozoic lithostratigraphy of the Jaisalmer Basin, Rajasthan. Journal of the Palaeontological Society of India , 52(2), 129–154.

52.

Singh

D. P.

, Saraswat

, Mohtadi

, & Kumar

(2022). Warm northern tropical Indian Ocean strengthened the ocean circulation prior to the last glacial termination. Global and Planetary Change , 209(103733). https://doi.org/10.1016/j.gloplacha.2021.103733

53.

Yan

, Shao

, Wang

, & Sun

(2013). Sr/Ca profile of long-lived Tridacna gigas bivalves from South China Sea: A new high-resolution SST proxy. Geochimica et Cosmochimica Acta , 112, 52–65. https://doi.org/10.1016/j.gca.2013.03.007