Developing a new paleosalinity proxy based on Na/Ca ratios in planktonic foraminifera
This project will develop a new upper ocean paleosalinity proxy based on Na/Ca ratios in two species of planktic foraminifera, Trilobatus sacculifer and Globigerinoides ruber, from a suite of sediment core tops spanning the subtropical/tropical Atlantic, Pacific, and Indian Oceans. Based on initial results from nine Atlantic core tops, salinity is the dominant factor controlling shell Na/Ca ratios in T. sacculifer (Watkins et al., 2021). However, the initial calibration needs to be expanded across a wider range of salinities and in other ocean basins. Because T. sacculifer is not always abundant in faunal assemblage at all locations, this project will also develop a calibration for G. ruber, a species with an even shallower depth habitat than T. sacculifer that is commonly utilized in studies reconstructing sea surface temperature (SST) and salinity (Lea et al., 2000; Schmidt et al., 2004; Schmidt and Lynch-Stieglitz 2011). The major goal of this project, therefore, is to develop species-specific global calibrations that can be used at any location. This project will also investigate and quantify the effects of dissolution on shell Na/Ca ratios in the Atlantic and Indian Oceans, as a recently published study showed that dissolution significantly impacts shell Na/Ca ratios in the tropical Pacific (Zhou et al., 2021). This will be achieved by measuring down-slope core tops from the tropical Atlantic and the south Indian Ocean. Finally, the newly developed calibrations will be used to generate a high-resolution record of Na/Ca-based salinity change in the Gulf Stream during Marine Isotope Stage 3 and compare it to a previously published salinity record based on the calculation of d18Oseawater using Mg/Ca-SST combined with d18O values in G. ruber (Schmidt et al., 2006). |
Reconstructing Mean State and ENSO Variability in the Eastern Equatorial Pacific under Glacial Forcing: A Combined Geochemical and Organic Proxy Approach
One of my current NSF funded projects involves reconstructing how the Eastern Equatorial Pacific (EEP) mean state and ENSO varied across the Dansgaard-Oeschger (D-O) cycles of Marine Isotope Stage 3 (MIS 3). The magnitude, duration, and number of these abrupt climate events make them the ideal natural experiment to test how the system will evolve in the near future. By utilizing a unique combination of multi-proxy methods together in a single study, this project will generate records of surface and subsurface temperature, thermocline temperature variance, upper-water column hydrography and upwelling variability from eight time slices in the EEP between 30 to 65 kyr. These objectives will be achieved by using a high-sedimentation rate core recovered from the heart of the cold-tongue EEP upwelling region during R/V Melville cruise MV1014 in 2010. Sedimentation rates in this core are among the highest in ENSO-sensitive regions of the tropical Pacific, ~10.5 cm/kyr, allowing for the resolution of millennial-scale climate events. The project will use a combination of foraminiferal stable isotope and trace metal geochemistry to reconstruct long-term changes in the EEP mean state across MIS 3. Next, 8 time slices spanning the extremes of MIS 3 climate (D-O interstadials, stadials and Heinrich Events) will be selected for further analyses. For each time slice, ENSO variability will be determined using thermocline temperature variance derived from single shell foraminiferal Mg/Ca analyses. In addition, upwelling intensity and nutrient variability will be characterized by the Diol Index and the Long Chain Diol Index. Together, this multi-proxy approach will allow for the most complete characterization of how the EEP varied across millennial-scale climate events of MIS 3 and will provide critical insight into how ENSO is related to extreme climate states of the past. This study will also provide climate modelers with critical information needed to simulate future climate change. |