Constraining past seawater δ 18 O and temperature records developed from foraminiferal geochemistry

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We present a detailed analysis of the seasonal distribution, size, morphological variability, and geochemistry of co‐occurring pink and white chromotypes of Globigerinoides ruber from a high‐resolution (1–2 weeks) and long‐running sediment trap time series in the northern Gulf of Mexico. We find no difference in the seasonal flux of the two chromotypes. Although flux of G. ruber is consistently lowest in winter, the flux‐weighted signal exported to marine sediments represents mean annual conditions in the surface mixed layer. We observe the same morphological diversity among pink specimens of G. ruber as white. Comparison of the oxygen and carbon isotopic composition (δ18O and δ13C) of two morphotypes (sensu stricto and sensu lato) of pink G. ruber reveals the isotopes to be indistinguishable. The test size distribution within the population varies seasonally, with the abundance of large individuals increasing (decreasing) with increasing (decreasing) sea surface temperature. We find no systematic offsets in the Mg/Ca and δ18O of co‐occurring pink and white G. ruber. The sediment trap data set shows that the Mg/Ca‐temperature sensitivity for both chromotypes is much lower than the canonical 9%/°C, which can likely be attributed to the secondary influence of both salinity and pH on foraminiferal Mg/Ca. Using paired Mg/Ca and δ18O, we evaluate the performance of a suite of published equations for calculating sea surface temperature, sea surface salinity, and isotopic composition of seawater (δ18Osw), including a new salinity‐δ18Osw relationship for the northern Gulf of Mexico from water column observations.

Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

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Considerations for Globigerinoides ruber (White and Pink) Paleoceanography: Comprehensive Insights From a Long‐Running Sediment Trap

Richey

Thirumalai

Khider

et al. 2019

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“…However, given the multivariate nature of Mg/Ca, it would be beneficial to use information from other temperature proxies in a formal hierarchical model structure. Previous work has already explored this avenue by combining Mg/Ca measurements with TEX 86 or Δ47 to estimate Mg/Ca sw (e.g., Evans et al, ; Evans, Brierley, et al, ; O'Brien et al, ) and by combining Mg/Ca with δ 18 O to infer δ 18 O sw or salinity (e.g., Oppo et al, ; Thirumalai et al, ; Tierney et al, ). Future work might explore incorporating δ 11 B and B/Ca estimates of pH and calcite saturation state, respectively.…”

Section: Discussionmentioning

confidence: 99%

Bayesian Calibration of the Mg/Ca Paleothermometer in Planktic Foraminifera

Tierney

Malevich

Gray

et al. 2019

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146

The Mg/Ca ratio of planktic foraminifera is a widely used proxy for sea-surface temperature but is also sensitive to other environmental factors. Previous work has relied on correcting Mg/Ca for nonthermal influences. Here, we develop a set of Bayesian models for Mg/Ca in four major planktic groups-Globigerinoides ruber (including both pink and white chromotypes), Trilobatus sacculifer, Globigerina bulloides, and Neogloboquadrina pachyderma (including N. incompta)-that account for the multivariate influences on this proxy in an integrated framework. We use a hierarchical model design that leverages information from both laboratory culture studies and globally distributed core top data, allowing us to include environmental sensitivities that are poorly constrained by core top observations alone. For applications over longer geological timescales, we develop a version of the model that incorporates changes in the Mg/Ca ratio of seawater. We test our models-collectively referred to as BAYMAG-on sediment trap data and on representative paleoclimate time series and demonstrate good agreement with observations and independent sea-surface temperature proxies. BAYMAG provides probabilistic estimates of past temperatures that can accommodate uncertainties in other environmental influences, enhancing our ability to interpret signals encoded in Mg/Ca. Plain Language SummaryThe amount of magnesium (Mg) incorporated into the calcite shells of tiny protists called foraminifera is determined by the temperature of the water in which they grew. This allows paleoclimatologists to measure the magnesium-to-calcium (Mg/Ca) ratio of fossil foraminiferal shells and determine how past sea-surface temperatures have changed. However, other factors can influence Mg/Ca, like the salinity and pH of seawater. Here, we develop Bayesian models of foraminiferal Mg/Ca that account for all of the influences on Mg/Ca and show how we can use these to improve our interpretations of Mg/Ca data. Culture experiments provide precise constraints on environmental sensitivities but are limited in that laboratory conditions are not perfect analogs for the natural environment. Sediment traps have an advantage in that seasonality of foraminiferal occurrence and corresponding ocean temperatures are well constrained, but they do not account for the effects of dissolution or bioturbation. Sedimentary core tops integrate effects associated with both occurrence and preservation and are thus better analogs for the conditions typical of the geological record, but uncertainties in seasonal preferences and the depth of calcification can in some cases lead to misleading inference of secondary environmental sensitivities (Hertzberg & Schmidt, 2013;Hönisch et al., 2013).Here, we use both core top and laboratory culture data to develop a suite of Bayesian hierarchical models for Mg/Ca. We collate over 1,000 sedimentary Mg/Ca measurements to formulate new calibrations for four major planktic groups: Globigerinoides ruber (including both pink and white chromotypes), Trilobat...

“…Chronological and proxy-related uncertainties associated with the T and IVC-δ 18 O sw time series from Site U1313 were probabilistically evaluated using a Monte Carlo-style approach in MATLAB (Marino et al, 2015;Thirumalai et al, 2016). For the T reconstructions, input data for the Monte Carlo routine are the samples' ages with their uncertainties related to the LR04 chronology (Lisiecki & Raymo, 2005), the G. ruber and G. crassaformis Mg/Ca values with their analytical errors, and the Mg/Ca-T calibrations with their uncertainties (Anand et al, 2003;Regenberg et al, 2009).…”

Section: Probabilistic Analysis Of Uncertaintiesmentioning

confidence: 99%

North Atlantic Midlatitude Surface‐Circulation Changes Through the Plio‐Pleistocene Intensification of Northern Hemisphere Glaciation

Bolton

Bailey

Friedrich

et al. 2018

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The North Atlantic Current (NAC) transports warm salty water to high northern latitudes, with important repercussions for ocean circulation and global climate. A southward displacement of the NAC and Subarctic Front, which separate subpolar and subtropical water masses, is widely suggested for the Last Glacial Maximum (LGM) and may have acted as a positive feedback in glacial expansion at this time. However, the role of the NAC during the intensification of Northern Hemisphere glaciation (iNHG) at ~3.5 to 2.5 Ma is less clear. Here we present new records from Integrated Ocean Drilling Program Site U1313 (41°N) spanning ~2.8–2.4 Ma to trace the influence of Subarctic Front waters above this mid‐latitude site. We reconstruct surface and permanent pycnocline temperatures and seawater δ18O using paired Mg/Ca‐δ18O measurements on the planktic foraminifers Globigerinoides ruber and Globorotalia crassaformis and determine abundances of the subpolar foraminifer Neogloboquadrina atlantica. We find that the first significant glacial incursions of Subarctic Front surface waters above Site U1313 did not occur until ~2.6 Ma. At no time during our study interval was (sub)surface reorganization in the midlatitude North Atlantic analogous to the LGM. Our findings suggest that LGM‐like processes sensu stricto cannot be invoked to explain interglacial‐glacial cycle amplification during iNHG. They also imply that increased glacial productivity at Site U1313 during iNHG was not only driven by southward deflections of the Subarctic Front. We suggest that nutrient injection from cold‐core eddies and enhanced glacial dust delivery may have played additional roles in increasing export productivity in the midlatitude North Atlantic from 2.7 Ma.