Earth's hydrological cycle was profoundly perturbed by massive carbon emissions during an ancient (56 Ma) global warming event referred to as the Paleocene‐Eocene thermal maximum (PETM). One approach to gaining valuable insight into the response of the hydrological cycle is to construct sea‐surface salinity (SSS) records that can be used to gauge changes in the rates of evaporation and precipitation during the PETM in such climatically sensitive areas as the circum‐Antarctic region. Here, we pair oxygen isotope (δ18O) and magnesium‐calcium (Mg/Ca) measurements to reconstruct PETM sea‐surface temperatures (SSTs) and δ18O composition of seawater (δ18Osw) at austral Site 690 (Weddell Sea). Several discrepancies emerge between the δ18O‐ and Mg/Ca‐based SST records, with the latter indicating that the earliest PETM was punctuated by a short‐lived ~4°C increase in local SSTs. Conversion of the δ18Oswvalues to SSS reveals a ~4 ppt decrease ~50 ka after peak PETM warming at Site 690. This negative SSS (δ18Osw) anomaly coincides with a prominent minimum in the planktic foraminifer δ18O record published for the Site 690 PETM section. Thus, our revised interpretation posits that this δ18O minimum signals a decrease in surface‐ocean δ18Oswfostered by a transient increase in mean annual precipitation in the Weddell Sea region. The results of this study corroborate the view that the poleward flux of atmospheric moisture temporarily increased during a distinctive stage of the PETM.
This content will become publicly available on August 1, 2025
- Award ID(s):
- 1405224
- PAR ID:
- 10531550
- Publisher / Repository:
- Paleoceanography and Paleoclimatology
- Date Published:
- Journal Name:
- Paleoceanography and Paleoclimatology
- Volume:
- 39
- Issue:
- 8
- ISSN:
- 2572-4517
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
more » « less
Abstract -
more » « less
Abstract A controversial aspect of Pliocene (5.3–2.6 Ma) climate is whether El Niño‐like (El Padre) conditions, characterized by a reduced trans‐equatorial sea‐surface temperature (SST) gradient, prevailed across the Pacific. Evidence for El Padre is chiefly based on reconstructions of sea‐surface conditions derived from the oxygen isotope (δ18O) and Mg/Ca compositions of shells belonging to the planktic foraminifer
Trilobatus sacculifer . However, fossil shells of this species are a mixture of multiple carbonate phases—pre‐gametogenic, gametogenic (reproductive), and diagenetic calcites—that formed under different physiological and/or environmental conditions and are averaged in conventional whole‐shell analyses. Through in situ measurements of micrometer‐scale domains within Pliocene‐aged shells ofT. sacculifer from Ocean Drilling Program Site 806 in the western equatorial Pacific, we show that the δ18O of gametogenic calcite is 0.6–0.8‰ higher than pre‐gametogenic calcite, while the Mg/Ca ratios of these two phases are the same. Both the whole‐shell and pre‐gametogenic Mg/Ca records indicate that average early Pliocene SSTs were ~1°C warmer than modern, with present‐day SSTs being established during the latest Pliocene and early Pleistocene (~3.0–2.0 Ma). The measurement of multiple calcite phases by whole‐shell δ18O analyses masks a late Pliocene to earliest Pleistocene (3.6–2.2 Ma) decrease in seawater δ18O (δ18Osw) values reconstructed from in situ pre‐gametogenic δ18O and Mg/Ca measurements. Our novel δ18Oswrecord indicates that sea‐surface salinities in the west Pacific warm pool were higher than modern prior to ~3.5 Ma, which is consistent with more arid conditions under an El Padre state. -
The early Eocene Climatic Optimum (EECO; ~ 53.3 to 49.1 Ma) was a period of the warmest sustained temperatures of the Cenozoic caused by perturbations to the global carbon cycle. Deep sea sediment cores and the microfossils preserved within them are the primary sources of information for these changes in climate and global carbon cycling but are subject to diagenetic alteration after deposition. One of the great challenges in paleoclimate research is determining how to accurately interpreting the proxy record by identifying the amount of chemical alteration of the isotopic and elemental compositions locked within microfossils such as foraminifera. The planktic foraminifera record has been biased by digenesis, provoking questions about the strength of the latitudinal temperature gradient throughout the EECO, specifically with respect to mismatches between proxy data and climate model simulations that remain unresolved. To investigate this question, we selected three deep sea sites that span across the Pacific Ocean, ODP Sites 865, 1209 and DSDP Site 207. From these sediments we extracted carefully screened planktic foraminifera and conducted analysis by two independent approaches on splits of the same individual foraminiferal shells. We measured the δ18O composition by conventional analysis (gas source mass spectrometry), and Mg/Ca ratios on fragments of the same shells by LA-ICP-MS that allows for a careful diagenetic screening. We then independently estimate sea surface temperatures and compare records to quantify the extent of bias in the planktonic foraminifera record. This approach helps to reassess the latitudinal temperature gradients across the EECO.more » « less
-
more » « less
Abstract Stable isotope data from tests of four planktonic foraminifer species in core tops on the Rio Grande Rise (RIO) fail a field test of reproducibility until corrections are made for various environmental effects. The regionally uniform and strongly stratified surface ocean hydrography across RIO allows the identification of causes of variability in δ18O and δ13C data in addition to the effects of surface ocean temperature and nutrient content. A previously calibrated calcite dissolution proxy indicates that the dissolution of foraminifer shells in sediments has no effect on δ18O and δ13C in tests of foraminifers from core tops on RIO. Furthermore, vital effects within and among foraminifer species are not sufficient to explain the large variability of δ18O and δ13C data observed on RIO. Instead, correctly estimating species‐specific habitat depth ranges and adjusting δ13C values for ocean/atmosphere carbon exchange are necessary to accurately reconstruct the hydrography of surface waters on RIO.
-
Planktic foraminiferal-based trace element-calcium ratios (TE/Ca) are a cornerstone in paleoceanographic reconstructions. While TE-environment calibrations are often established through culturing experiments, shell growth in culture is not always consistent with growth in a natural setting. For example, many species of planktic foraminifera thicken their shell at the end of their life cycle, producing a distinct “gametogenic” crust. Crust is common in fossil foraminifers, however, shells grown in culture do not often develop a thick crust. Here, we investigate potential vital effects associated with the crusting process by comparing the trace element (Mg/Ca, Na/Ca, Ba/Ca, Sr/Ca, Mn/Ca, Zn/Ca) and stable isotope (δ13C, δ18O) composition of alive, fully mature, uncrusted shells to recently deceased, crusted shells of Neogloboquadrina pachyderma collected from the same plankton tows off the Oregon (USA) coast. We find that uncrusted (N = 55) shells yield significantly higher Ba/Ca, Na/Ca, Mn/Ca, and Sr/Ca than crusted (N = 66) shells, and crust calcite records significantly lower TE/Ca values for all elements examined. Isotopic mixing models suggest that the crust calcite accounts for ∼40%–70% of crusted shell volume. Comparison of foraminiferal and seawater isotopes indicate that N. pachyderma lives in the upper 90 m of the water column, and that crust formation occurs slightly deeper than their average living depth habitat. Results highlight the necessity to establish calibrations from crusted shells, as application of calibrations from TE-enriched uncrusted shells may yield attenuated or misleading paleoceanographic reconstructions.more » « less
