The Western Equatorial Pacific (WEP) warm pool, with surface temperatures >28
El Niño Southern Oscillation (ENSO) is the largest source of interannual climate variability on Earth today; however, future ENSO remains difficult to predict. Evaluation of paleo‐ENSO may help improve our basic understanding of the phenomenon and help resolve discrepancies among models tasked with simulating future climate. Individual foraminifera analysis allows continuous down‐core records of ENSO‐related temperature variability through the construction and comparison of paleotemperature distributions; however, there has been little focus on calibrating this technique to modern conditions. Here, we present data from individual measurements of Mg/Ca in two species of planktic foraminifera, surface dwelling
- NSF-PAR ID:
- 10457936
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Paleoceanography and Paleoclimatology
- Volume:
- 35
- Issue:
- 2
- ISSN:
- 2572-4517
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract ° C and a deep thermocline, is an important source of latent and sensible heat for the global climate system. Because the tropics are not sensitive to ice‐albedo feedbacks, the WEP's response to radiative forcing can be used to constrain a minimum estimate of Earth system sensitivity. Climate modeling ofp CO2‐radiative warming projections shows little change in WEP variability; here we use temperature distributions of individual surface and subsurface dwelling fossil foraminifera to evaluate past variability and possible radiative and dynamic climate forcing over the Plio‐Pleistocene. We investigate WEP warm pool variability within paired glacial‐interglacial (G‐IG) intervals for four times: the Holocene‐Last Glacial Maximum, ~2, ~3, and ~4 Ma. Our results show that these surface and subsurface temperature distributions are similar for all G‐IG pairs, indicating no change in variability, even asp CO2‐radiative forcing and other boundary conditions changed on G‐IG timescales. Plio‐Pleistocene sea surface temperature (SST) distributions are similar to those from the Holocene, indicating WEP SSTs respond top CO2‐radiative forcing and associated feedbacks. In contrast, Plio‐Pleistocene subsurface temperature distributions suggest subsurface temperatures respond to changes in thermocline temperature and depth. We estimate tropical temperature sensitivity for the mid‐Pliocene (~3 Ma) using our individual foraminifera SST data set and a previously published high‐resolution boron isotope‐basedp CO2reconstruction. We find tropical temperature sensitivity was equal to, or less than, that of the Late Pleistocene. -
ABSTRACT The trace element composition of planktic foraminifera shells is influenced by both environmental and biological factors (‘vital effects’). As trace elements in individual foraminifera shells are increasingly used as paleoceanographic tools, understanding how trace element ratios vary between individuals, among species, and in response to high frequency environmental variability is of critical importance. Here, we present a three-year plankton tow record (2010–2012) of individual shell trace element (Mg, Sr, Ba, and Mn) to Ca ratios in the planktic species Globigerina ruber (pink), Orbulina universa, and Globorotalia menardii collected throughout the upper 100 m of Cariaco Basin. Plankton tows were paired with in situ measurements of water column chemistry and hydrography. The Mg/Ca ratio reflects different calcification temperatures in all three species when calculated using species-specific temperature relationships from single-species averages of Mg/Ca. However, individual shell Mg/Ca often results in unrealistic temperate estimates. The Sr/Ca ratios are relatively constant among the four species. Ratios of Mn/Ca and Ba/Ca are highest in G. menardii and are not reflective of elemental concentrations in open waters. The Mn/Ca ratio is elevated in all species during upwelling conditions, and a similar trend is demonstrated in Neogloboquadrina incompta shells from the California margin collected during upwelling periods. Together this suggests that elevated shell Mn/Ca may act as a tracer for upwelling of deeper water masses. Our results emphasize the large degree of trace element variability present among and within species living within a limited depth habitat and the roles of biology, calcification environment, and physical mixing in mediating how trace element geochemistry reflects environmental variability in the surface ocean.more » « less
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Abstract Coral Sr/Ca records have been widely used to reconstruct and understand past sea surface temperature (SST) variability in the tropical Pacific. However, in the eastern equatorial Pacific, coral growth conditions are marginal, and strong El Niño events have led to high mortality, limiting opportunities for coral Sr/Ca‐based SST reconstructions. In this study, we present two ∼25‐year Sr/Ca and Mg/Ca records measured on modern
Porites lobata from Wolf and Darwin Islands in the northern Galápagos. In these records, we confirm the well‐established relationship between Sr/Ca and SST and investigate the impact of heat stress on this relationship. We demonstrate a weakened relationship between Sr/Ca and SST after a major (Degree Heating Months 9°C‐months) heat stress event during the 1997–1998 El Niño, with a larger response in the Wolf core. However, removing data that covers the 1997–1998 El Niño from calibration does not improve reconstruction statistics. Nevertheless, we find that excluding dataafter the 1997–1998 El Niño event from the calibration reduces the SST reconstruction error slightly. These results confirm that coral Sr/Ca is a reliable SST proxy in this region, although it can respond adversely to unusual heat stress. We suggest that noise in Sr/Ca‐SST calibrations may be reduced by removing data immediately following large heat extremes. -
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Abstract Establishing tropical sea surface temperature (SST) during the Last Glacial Maximum (LGM) is important for constraining equilibrium climate sensitivity to radiative forcing. Until now, there has been little data from the central equatorial Pacific in global compilations, with foraminiferal assemblage‐based estimates suggesting the region was within 1°C of modern temperatures during the LGM. This is in stark contrast to multi‐proxy evidence from the eastern and western Pacific and model simulations which support larger cooling. Here we present the first estimates of glacial SST in the central equatorial Pacific from Mg/Ca in
Globigerinoides ruber . Our results show that the central Pacific cooled by about 2.0°C during the LGM, in contrast with previous global compilations but in agreement with models. Our data support a larger magnitude of tropical LGM cooling, and thus a larger equilibrium climate sensitivity, than previous studies which relied on foraminiferal assemblages in the central tropical Pacific. -
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Abstract Foraminiferal Mg/Ca has proven to be a powerful paleothermometer for reconstructing past sea‐surface temperature, which, among other applications, is a critical parameter for boron isotope reconstructions of past surface ocean pH and PCO2. However, recent laboratory culture studies indicate seawater pH and the total dissolved inorganic carbon content (DIC) may both exert a significant additional control on foraminiferal Mg/Ca, likely influencing paleotemperature records as a result of seawater chemistry evolution on geologic timescales. In addition, the seawater Mg/Ca composition (Mg/Casw) has been shown to reduce the sensitivity of foraminiferal Mg/Ca to temperature and possibly its sensitivity to the carbonate system as well. Here we present new Mg/Ca data from laboratory culture experiments with living planktic foraminifera—
Globigerinoides ruber (p),Trilobatus sacculifer , andOrbulina universa — grown under a range of different pH and/or seawater DIC conditions and in low Mg/Caswto mimic the chemical composition of the Paleocene ocean. We also conducted targeted [Ca] experiments to help define Mg/Cacalcite–Mg/Caswrelationships for each species and conducted new pH experiments withG .bulloides . We find that pH effects on foraminiferal Mg/Ca are reduced or absent at Mg/Casw = 1.5 mol/mol in all three species, and thatT .sacculifer is generally insensitive to variable DIC and pH, making it the ideal species for Mg/Ca SST reconstructions back to 20 Ma. We apply our newT .sacculifer calibration to a Middle Miocene Mg/Ca record and provide recommendations for interpreting Mg/Ca records from extinct species.
