Coupled Mg/Ca and Clumped Isotope Measurements Indicate Lack of Substantial Mixed Layer Cooling in the Western Pacific Warm Pool During the Last ∼5 Million Years

The Indo-Pacific Warm Pool (IPWP) plays a crucial role in influencing climate dynamics both in the tropics and globally. Yet, there is an ongoing controversy concerning the evolution of surface temperatures in the IPWP since the Pliocene, which is fueled by contradictory proxy evidence. Temperature reconstructions using TEX86 indicate a gradual cooling by ∼2°C from the Pliocene to today while Mg/Ca-based studies using planktonic foraminifera do not report any long-term trends. A bias in Mg/Ca records due to seawater chemistry changes has been suggested as an explanation for this proxy mismatch. Here, we present data from two independent foraminifera-based temperature proxies, Mg/Ca and clumped isotopes (Δ47), measured on the same samples from IODP Site U1488 in the IPWP. We reconstructed mixed layer and subsurface temperatures and find very good agreement among Mg/Ca and Δ47 when applying a minor correction for changing Mg/Ca ratios of seawater. Diagenetic effects could influence Δ47 but the evaluation of foraminifera preservation at Site U1488 suggests that this effect is unlikely to have masked a long-term trend in the data. While remaining uncertainties prevent us from fully ruling out particular hypotheses, our study adds evidence that mixed layer temperatures likely did not cool substantially, while subsurface temperatures cooled more strongly since the Pliocene. The substantial Pleistocene cooling previously observed in TEX86 data is consistent with this finding when interpreting it as a combined surface and subsurface signal.publishedVersio

Clumped isotope thermometry in foraminifera - From calibration to Plio-Pleistocene temperature reconstructions in the Indo-Pacific Warm Pool

Clumped isotopes thermometry on foraminifera holds the potential to accurately reconstruct ocean temperatures on million-year timescales. In contrast to most other paleothermometers, clumped isotopes do not rely on prior knowledge regarding ocean chemistry changes thus evading a major source of uncertainty inherent to most other paleothermometers. This thesis aims to amend our understanding of the clumped isotope signal in foraminifera and provide improvements to this paleothermometer for application to ocean sediments (Paper I). The knowledge gained is applied to Plio-Pleistocene sequences from two locations within the Indo-Pacific Warm Pool in order to address discrepancies among other temperature proxies and shed new light on long-standing debates regarding the long-term temperature evolution of this crucial region in the global ocean (Papers II and III). The results displayed in this thesis include a clumped isotope to temperature calibration dataset for planktonic foraminifera that was combined with several existing foraminifer-based calibrations in order to elucidate potential laboratory differences as well as species effects on the paleothermometer (Paper I). Our combined calibration highlights the excellent agreement among various analytical approaches and different foraminifera species. This work thus provides a robust tool to reconstruct past ocean temperatures using various species and on million-year time scales. The results of papers II and III constrain the Plio-Pleistocene temperature evolution of the Indo-Pacific Warm Pool independent of ocean chemistry changes. The knowledge gained can be used to disentangle the influences of these reservoir changes and the temperature signal recorded in other proxies. The paired Mg/Ca and clumped isotope records comprise evidence for the validity of Plio-Pleistocene Mg/Ca-based sea surface temperature reconstructions and argue against a systematic bias of Pliocene Mg/Ca temperature estimates by past Mg/Ca changes of seawater. The long-term temperature evolution of the Indo-Pacific Warm Pool across the Plio-Pleistocene is illustrated by the combined mixed layer and thermocline records from IODP Sites U1488 (Paper II), U1482 and U1483 (Paper III). The evidence presented in viii this thesis portrays a vertical and lateral expansion of the Indo-Pacific Warm Pool during the Pliocene relative to its modern extent, albeit without significant warming of surface waters in the central part. Our results document the strengthening of glacial periods from the Pliocene to present, while interglacial temperatures are not subject to long-term trends. Moreover, our results corroborate Mg/Ca records from the equatorial Pacific and thus support the hypothesis of a “permanent El Niño-like state” or “El Padre” during the Early Pliocene. The potential of clumped isotope thermometry on foraminifera for the reconstruction of past ocean temperatures at different depths in the water column is demonstrated by the results included in this thesis. Applied on Cenozoic time scales this paleothermometer ideally complements other, higher resolution methods by providing the opportunity to test these proxies and deliver independent constraints on non-thermal effects such as ocean chemistry changes

Clumped isotope thermometry in foraminifera - From calibration to Plio-Pleistocene temperature reconstructions in the Indo-Pacific Warm Pool

Clumped isotopes thermometry on foraminifera holds the potential to accurately reconstruct ocean temperatures on million-year timescales. In contrast to most other paleothermometers, clumped isotopes do not rely on prior knowledge regarding ocean chemistry changes thus evading a major source of uncertainty inherent to most other paleothermometers. This thesis aims to amend our understanding of the clumped isotope signal in foraminifera and provide improvements to this paleothermometer for application to ocean sediments (Paper I). The knowledge gained is applied to Plio-Pleistocene sequences from two locations within the Indo-Pacific Warm Pool in order to address discrepancies among other temperature proxies and shed new light on long-standing debates regarding the long-term temperature evolution of this crucial region in the global ocean (Papers II and III). The results displayed in this thesis include a clumped isotope to temperature calibration dataset for planktonic foraminifera that was combined with several existing foraminifer-based calibrations in order to elucidate potential laboratory differences as well as species effects on the paleothermometer (Paper I). Our combined calibration highlights the excellent agreement among various analytical approaches and different foraminifera species. This work thus provides a robust tool to reconstruct past ocean temperatures using various species and on million-year time scales. The results of papers II and III constrain the Plio-Pleistocene temperature evolution of the Indo-Pacific Warm Pool independent of ocean chemistry changes. The knowledge gained can be used to disentangle the influences of these reservoir changes and the temperature signal recorded in other proxies. The paired Mg/Ca and clumped isotope records comprise evidence for the validity of Plio-Pleistocene Mg/Ca-based sea surface temperature reconstructions and argue against a systematic bias of Pliocene Mg/Ca temperature estimates by past Mg/Ca changes of seawater. The long-term temperature evolution of the Indo-Pacific Warm Pool across the Plio-Pleistocene is illustrated by the combined mixed layer and thermocline records from IODP Sites U1488 (Paper II), U1482 and U1483 (Paper III). The evidence presented in viii this thesis portrays a vertical and lateral expansion of the Indo-Pacific Warm Pool during the Pliocene relative to its modern extent, albeit without significant warming of surface waters in the central part. Our results document the strengthening of glacial periods from the Pliocene to present, while interglacial temperatures are not subject to long-term trends. Moreover, our results corroborate Mg/Ca records from the equatorial Pacific and thus support the hypothesis of a “permanent El Niño-like state” or “El Padre” during the Early Pliocene. The potential of clumped isotope thermometry on foraminifera for the reconstruction of past ocean temperatures at different depths in the water column is demonstrated by the results included in this thesis. Applied on Cenozoic time scales this paleothermometer ideally complements other, higher resolution methods by providing the opportunity to test these proxies and deliver independent constraints on non-thermal effects such as ocean chemistry changes

Coupled Mg/Ca and Clumped Isotope Measurements Indicate Lack of Substantial Mixed Layer Cooling in the Western Pacific Warm Pool During the Last ∼5 Million Years

The Indo-Pacific Warm Pool (IPWP) plays a crucial role in influencing climate dynamics both in the tropics and globally. Yet, there is an ongoing controversy concerning the evolution of surface temperatures in the IPWP since the Pliocene, which is fueled by contradictory proxy evidence. Temperature reconstructions using TEX86 indicate a gradual cooling by ∼2°C from the Pliocene to today while Mg/Ca-based studies using planktonic foraminifera do not report any long-term trends. A bias in Mg/Ca records due to seawater chemistry changes has been suggested as an explanation for this proxy mismatch. Here, we present data from two independent foraminifera-based temperature proxies, Mg/Ca and clumped isotopes (Δ47), measured on the same samples from IODP Site U1488 in the IPWP. We reconstructed mixed layer and subsurface temperatures and find very good agreement among Mg/Ca and Δ47 when applying a minor correction for changing Mg/Ca ratios of seawater. Diagenetic effects could influence Δ47 but the evaluation of foraminifera preservation at Site U1488 suggests that this effect is unlikely to have masked a long-term trend in the data. While remaining uncertainties prevent us from fully ruling out particular hypotheses, our study adds evidence that mixed layer temperatures likely did not cool substantially, while subsurface temperatures cooled more strongly since the Pliocene. The substantial Pleistocene cooling previously observed in TEX86 data is consistent with this finding when interpreting it as a combined surface and subsurface signal

Mg/Ca, Sr/Ca, Mn/Ca ratios measured in planktonic foraminifera species T. trilobus and G. tumida from IODP Site U1488 and temperature reconstructions from 5.6 Ma to present

This dataset contains Mg/Ca, Sr/Ca and Mn/Ca data from IODP Site U1488 in the central Western Pacific Warm Pool measured in two species of planktonic foraminifera, Trilobatus trilobus and Globorotalia tumida. The trace element data was generated at the University of California, Santa Cruz (UCSC) using a Thermo Fisher iCAP 7400 ICP-OES. The dataset also contains water temperature reconstructions for both species based on the Mg/Ca data applying various corrections and using published Mg/Ca to temperature calibrations. The data covers eight time intervals from the last 5.6 Myrs. Trace element and Mg/Ca-based temperature data are presented for each sample within these time intervals. Additionally, for each of the time intervals average calcification temperatures based on clumped isotope data and Mg/Ca ratios were calculated. Clumped isotope measurements were performed at the University of Bergen, Norway, using a Thermo Scientific MAT 253Plus mass spectrometer coupled to a KIEL IV carbonate device

Application of Clumped Isotope Thermometry to Benthic Foraminifera

Obtaining absolute temperatures of the ocean in deep time is complicated by the lack of constraints on seawater chemistry. Seawater salinity, carbonate ion concentration, δ18O, and elemental abundance changes may obscure widely applied paleoproxies. In addition, with foraminifera‐based proxies applied over long time scales or through major transitions, taxonomic turnover can impair the robustness of a record. While requiring larger sample sizes than most other proxies, the clumped isotope method is independent of seawater chemistry. Here we test if small benthic foraminifera precipitate their carbonate in equilibrium with respect to the clumped isotope thermometer and if there are any species‐specific vital effects. We find that benthic foraminifera fall on the same calibration line as the majority of carbonate minerals including inorganic calcite. In addition, we find no offsets that can be attributed to a species‐specific for any of the samples. This finding implies that a necessary amount of sample material can be obtained by aggregating over multiple taxa of benthic foraminifera and allows for the application of this proxy over major climatic transitions that coincide with seawater chemistry changes and foraminifera extinctions

International Ocean Discovery Program Expedition 363 Preliminary Report Western Pacific Warm Pool Neogene and Quaternary records of Western Pacific Warm Pool paleoceanography

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