PhD Student Alex Reis sailed on IODP Leg 378- South Pacific Paleogene Climate and this project will constitute the majority of his dissertation
Subsurface Redox Chemistry and Dolomite Formation on the Campbell Plateau: Implications for Paleocene Environmental Reconstructions at IODP Site 378-U1553
Lithostratigraphic column showing the recovery at Site U1553 along with relevent geochemical results. Left to right: pore water sulfate, sedimentary sulfur, headspace methane. Modified from Thomas et al., 2020. Sample intervals for the proposed research outlined in red on the stratigraphic column. Interval for my ongoing research are highlighted in blue.
Dolomitization of marine carbonate sediments can provide insight into regional burial conditions and redox chemistry. These minerals record the conditions of the sediment and fluid chemistry during their formation. By combining records from the dolomite with chemical analyses of the surrounding sediment, the timing and mechanisms leading to dolomite formation can be identified. This study will analyze multiple isotopic indicators to provide complementary lines of evidence for the driving mechanism and timing of dolomitization. The mechanism and timing of dolomite formation has implications for paleoenvironmental reconstructions, indicating deposition in a restricted basin and/or burial processes which could overprint the primary record.
Assessing sedimentary detrital Pb isotopes as a dust tracer in the Pacific Ocean
Andrea M. Erhardt, Grant Douglas, Andrew D. Jacobson, Josh Wimpenny, Qing-Zhu Yin, Adina Paytan
206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb results from core top sediment detrital fractions across the Pacific Ocean measured for this study (outlined in dark circles) and published data (Godfrey, 2002; Hyeong et al., 2011; Jones et al., 2000; Pettke et al., 2000; Stancin et al., 2006, 2008; Xie and Marcantonio, 2012). Warmer colors represent more radiogenic ratios, and cooler colors less radiogenic ratios. Results for source regions are listed on the figure. 1Published results from Stancin et al. (2006) calculated from analysis of core top data; 2Published ratios measured on the silicate (detrital) fraction of Chinese loess by Jones et al. (2000); 3Published results of pre-anthropogenic Saharan dust compiled by Abouhami and Zabel (2003); 3Published results for Papua New Guinea from Park et al. (2010). ITCZ summer (red) and winter (green) from Liu et al. (2015).
Erhardt, A.M., Douglas, G., Jacobson, A.D., Wimpenny, J., Yin, Q-Z., Paytan, A. (2021). Assessing sedimentary detrital Pb isotopes as a dust tracer in the Pacific Ocean. Paleoceanography and Paleoclimatology, 36 (4), e2020PA004144. DOI: 10.1029/2020PA004144
Key Points:
1) Pb isotopic ratios of detrital fractions provide general trends but not definitive determination of dust source regions
2) Mixing models are sensitive to small changes in source isotopic values, highlighting importance of accurate source characterization
3) Glacial/Interglacial downcore variability in Pb isotopic ratios reflect climate-driven dust delivery to the Eastern Equatorial Pacific
Abstract
Mineral dust particles from different source regions typically have distinct Pb isotope ratios. Theoretically, Pb isotopic composition of terrigenous minerals isolated from open-ocean sediments should allow for dust provenance reconstructions. However, Pb isotopes of terrigenous fractions of sediments have frequently been inconsistent with expected source region signatures. This study investigates the reason(s) for offsets between the Pb isotope values of the dust component in sediment cores and those expected from source regions with focus on changes in sediment composition, sediment age and sediment processing for analysis.
Pb isotope ratios from Pacific Ocean core top sediments show a general delineation of the Intertropical Convergence Zone (ITCZ). Isotope mixing models support these general trends, though similarity in Pb isotope ratios of disparate source regions makes constraining specific sources challenging. Pb isotope ratios in down core samples varied on glacial/interglacial time scales, being less radiogenic during the last glacial maximum, suggesting either a weakened ITCZ or the addition of a new, less radiogenic, source to the system. Finally, Pb isotope ratios in some source region samples yielded different Pb isotope signatures in bulk source sample than in the insoluble terrigenous fraction of the source sample, indicating that differential mineral preservation within the terrigenous component in sediments may cause offsets from source signatures. Overall, while Pb isotopes show distinct basin-scale variations, high-resolution spatial reconstructions require tight age controls and consistency in analytical treatment if used to define ocean sediment source regions.