An official website of the United States government
Abstract We explore the response of northeastern Pacific sea surface temperature (SST) to deglacial (16–7 ka) climate variability as recorded in‐based SST reconstructions spanning 65°N to 10°S. Included in the analysis is a new 23 kyr SST record from core NH8P from the northwest Mexican Margin. We isolate spatiotemporal patterns in regional SSTs with trend empirical orthogonal function (TEOF) analysis. The dominant TEOF mode reflects deglacial warming associated with rising. Tropical and subtropical SSTs correlated most strongly with this mode, suggesting that the thermodynamic response of the tropical eastern Pacific to greenhouse gas forcing was the dominant driver of regional SST change during deglaciation. The second TEOF mode reflects millennial‐scale variability and is most strongly expressed in subpolar SSTs. The synchronous timing between North Pacific and North Atlantic SST oscillations is evidence for the rapid transmission of millennial‐scale climate perturbations between the basins, likely through an atmospheric teleconnection. SSTs at NH8P have no correlation with either leading TEOF mode as there is minimal change in SST at this site after20 ka. A model simulation of the LGM indicates that glacial cooling was muted in much of the Eastern Pacific Warm Pool (EPWP), in which NH8P lies, due to reductions in latent heat flux. This suggests that the wind‐evaporation‐SST feedback was responsible for the attenuation of EPWP cooling. Overall, this study highlights the distinct latitudinal trends in the Pacific's response to deglaciation.
more »
« less
Impacts of Paleoecology on the TEX 86 Sea Surface Temperature Proxy in the Pliocene‐Pleistocene Mediterranean Sea
Abstract The TEX86proxy, based on the distribution of isoprenoid glycerol dialkyl glycerol tetraethers (iGDGTs) from planktonic Thaumarchaeota, is widely used to reconstruct sea surface temperature (SST). Recent observations of species‐specific and regionally dependent TEX86‐SST relationships in cultures and the modern ocean raise the question of whether nonthermal factors may have impacted TEX86paleorecords. Here we evaluate the effects of ecological changes on TEX86using one Pliocene and two Pleistocene sapropels from the Mediterranean Sea. We find that TEX86‐derived SSTs deviate from‐derived SSTs before, during, and after each sapropel event.‐derived SSTs vary by less than 6 °C, while TEX86‐derived SSTs vary by up to 15 °C within a single record. Compound‐specific carbon isotope compositions indicate minimal confounding influence on TEX86from exogenous sources. Some of the variation can be accounted for by changes in nitrogen cycling intensity affecting thaumarchaeal iGDGT biosynthesis, as demonstrated by an inverse relationship between TEX86and δ15NTN. TEX86‐derived SSTs also consistently show warm anomalies in the Pleistocene, while the Pliocene samples exhibit both warmer and cooler relative offsets. These anomalies result from systematic differences between Plio‐Pleistocene iGDGT distributions and both modern Mediterranean and modern, globally distributed core top samples. Through characteristic GDGT distributions, we suggest the existence of three distinct endemic populations of Thaumarchaeota in the Pliocene, Pleistocene, and modern Mediterranean Sea, respectively. Importantly, these communities prevailed during both sapropel and oligotrophic conditions. Our results demonstrate that ecological and community‐specific effects must be considered when applying the TEX86proxy to paleorecords.
more »
« less
- Award ID(s):
- 1702262
- PAR ID:
- 10462809
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Paleoceanography and Paleoclimatology
- Volume:
- 33
- Issue:
- 12
- ISSN:
- 2572-4517
- Page Range / eLocation ID:
- p. 1472-1489
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Future projections of southwestern African hydroclimate are highly uncertain. However, insights from past warm climates, like the Pliocene, can reveal mechanisms of future change and help benchmark models. Using leaf wax hydrogen isotopes to reconstruct precipitation (δDp) from Namibia over the past 5 million years, we find a long‐term depletion trend (−50‰). Empirical mode decomposition indicates this trend is linked to sea surface temperatures (SSTs) within the Benguela Upwelling System, but modulated by Indian Ocean SSTs on shorter timescales. The influence of SSTs on reconstructed regional hydroclimate is similar to that observed during modern Benguela Nio events, which bring extreme flooding to the region. Isotope‐enabled simulations and PlioMIP2 results suggest that capturing a Benguela Nio‐like state is key to accurately simulating Pliocene, and future, regional hydroclimate. This has implications for future regional climate, since an increased frequency of Benguela Nios poses risk to the ecosystems and industries in the region.more » « less
-
Abstract Upwelling deep waters in the Southern Ocean release biologically sequestered carbon into the atmosphere, contributing to the relatively high atmospheric CO2levels during interglacial climate periods. Paleoceanographic evidence suggests this “CO2leak” was lessened during the last glacial maximum (LGM), potentially due to increased stratification, weaker and equatorward‐shifted winds, and/or enhanced biological carbon export. The collective influences of these mechanisms on the ocean's biological pump efficiency and amount of atmospheric CO2can be quantified by determining preformed phosphate of deep waters. We quantify preformed PO4(Ppre,AOU) and preformed() of LGM bottom waters using a compilation of published paleo‐temperature, nutrient and oxygen estimates from benthic foraminifera. Our results show that preformed phosphate of the Pacific and Indian deep oceans was reduced by about −0.53 ± 0.13 μM and suggest that much (64 ± 28 ppmv) of the Glacial‐Interglacial CO2drawdown resulted from changes in the ocean's biological pump efficiency. Once carbonate compensation is accounted for, this can explain the entire CO2drawdown (87 ± 40 ppmv). Preformedshows similar results. The reconstructed LGM Ppre,AOUand oxygen are qualitatively consistent with the changes produced by a suite of numerical sensitivity experiments that roughly simulate three proposed mechanisms for an increase in LGM biological pump efficiency: an increase in biological activity, a decrease in wind‐driven upwelling, and an increase in stratification in the Southern Ocean.more » « less
-
Abstract The meridional variability of the Subtropical Front (STF) in the Southern Hemisphere, linked to expansions or contractions of the Southern Ocean, may have played an important role in global ocean circulation by moderating the magnitude of water exchange at the Indian‐Atlantic Ocean Gateway, so called Agulhas Leakage. Here we present new biomarker records of upper water column temperature (and) and primary productivity (chlorins and alkenones) from marine sediments at IODP Site U1475 on the Agulhas Plateau, near the STF and within the Agulhas retroflection pathway. We use these multiproxy time‐series records from 1.4 to 0.3 Ma to examine implied changes in the upper oceanographic conditions at the mid‐Pleistocene transition (MPT, ca. 1.2–0.8 Ma). Our reconstructions, combined with prior evidence of migrations of the STF over the last 350 ka, suggest that in the Southwestern Indian Ocean the STF may have been further south from the Agulhas Plateau during the mid‐Pleistocene Interim State (MPIS, MIS 23–12) and reached its northernmost position during MIS 34–24 and MIS 10. Comparison to aGloborotalia menardii‐derived Agulhas Leakage reconstruction from the Cape Basin suggests that only the most extreme northward migrations of the STF are associated with reduced Agulhas Leakage. During the MPIS, STF migrations do not appear to control Agulhas Leakage variability, we suggest previously modeled shifting westerly winds may be responsible for the patterns observed. A detachment between STF migrations and Agulhas Leakage, in addition to invoking shifting westerly winds may also help explain changes in CO2ventilation seen during the MPIS.more » « less
-
Abstract Global ocean mean salinityis a key indicator of the Earth's hydrological cycle and the exchanges of freshwater between land and ocean, but its determination remains a challenge. Aside from traditional methods based on gridded salinity fields derived from in situ measurements, we explore estimates ofbased on liquid freshwater changes derived from space gravimetry data corrected for sea ice effects. For the 2005–2019 period analyzed, the differentseries show little consistency in seasonal, interannual, and long‐term variability. In situ estimates show sensitivity to choice of product and unrealistic variations. A suspiciously large rise insince ∼2015 is enough to measurably affect halosteric sea level estimates and can explain recent discrepancies in the global mean sea level budget. Gravimetry‐basedestimates are more realistic, inherently consistent with estimated freshwater contributions to global mean sea level, and provide a way to calibrate the in situ estimates.more » « less
