Browsing by Author "Bhattacharya, Joyeeta"

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    Marine carbonate accumulation during the greenhouse climate of the Eocene
    (2021-04-28) Bhattacharya, Joyeeta; Yeung, Laurence Y.
    Major climatic transitions and aberrations accompanied by global carbon cycle perturbations are known to have occurred during the Eocene epoch, between 56-34 Ma. The warmest interval of the Cenozoic culminated during early Eocene, punctuated by multiple events of rapid injection of isotopically light carbon into the ocean-atmosphere system, e.g. Paleocene Eocene Thermal Maximum (PETM), ca. 56 Ma, which were preserved as negative carbon isotope excursions (CIEs) in the sedimentary record. The hothouse climate of early Eocene transitioned to a relatively cooler and less explored ‘warmhouse’ regime during middle-late Eocene, when multiple transient warming events occurred, which were at times associated with significant fluctuations in the oceanic carbonate saturation horizons. This study documents changes in carbonate dissolution indices and carbonate mass accumulation rate through the entire Eocene (56–34 Ma) at ODP Site 1209 on Shatsky Rise, north-central Pacific and ties it to a stable carbon and oxygen isotope record, which is stratigraphically correlated with the global benthic stack and equatorial Pacific CCD reconstruction of the Eocene, in order to explore the links between climate change, carbon cycling and marine carbonate accumulation. Our study finds a strong correlation between magnitude of CIEs and intensity of dissolution for early Eocene hyperthermal events which appears to significantly weaken for multiple dissolution events in middle-late Eocene, possibly indicating fundamental difference in their causal mechanism. As this study and multiple others in paleoceanography utilize bulk sediment carbon isotope (δ13C) records in Cenozoic chemostratigraphy; it is crucial to deconvolute the isotope signal of marine bulk sediments. Pelagic carbonates are ensemble of different components of varying proportion and distinct isotope composition; hence spatial and temporal co-variation of pelagic carbonate constituents is important to understand in order to make a comprehensive interpretation of bulk-δ13C records. In this part of the study, sediments from the late Paleocene-early Eocene interval (58-50 Ma), at ODP Site 1209 are dissected into size fractions to investigate how a temporally varying bulk carbonate ensemble influences the overall carbon isotope record. A set of 45 samples were examined for their bulk and size-fraction-specific δ13C and δ18O compositions. We find a significant increase in coarse-fraction abundance across PETM, driven by a changing community structure of calcifiers, modulating the size of CIE at Site 1209 and thus making it distinct from those recorded at other open-marine sites. These results highlight the importance of biogeography in marine stable-isotope record, especially when isotope excursions are driven by climate- and/or carbon-cycle changes. In addition, community composition changes will alter the interpretation of weight percent coarse fraction as a conventional proxy for carbonate dissolution. The next part of the work focusses on chemostratigraphy of the Lutetian-Bartonian stages of middle Eocene from sites on Tasman Sea, in the southwest Pacific, which represent a ‘warmhouse’ climate state: a transition from supergreenhouse/hothouse climate of early Eocene to the onset of polar glaciation and ‘coolhouse’ climate of Oligocene. Despite long-term cooling conditions in the background, middle Eocene was punctuated by at least one significant global warming and sea-floor carbonate dissolution event, referred to as the Middle Eocene Climate Optimum (MECO), ca. ~40.5 Ma. Over the last decade, studies from the Atlantic deep ocean sites have identified at least one other brief event of carbon cycle perturbation, during polarity chron 19R; referred to as Late Lutetian Thermal Maximum (LLTM). But its global extent remains unresolved. Our current work develops a stable isotope stratigraphy of middle Eocene sediments, precisely within the late Lutetian – early Bartonian stages from four different sites across Tasman Sea, two of which were drilled during IODP Expedition 371 (U1508C and U1509A) and two of which derive from field work in New Caledonia (the Sommet-Khian outcrop and the Cadart drill core). Here we identify long-term changes in carbon and oxygen isotope records, possibly related to 405 Ky eccentricity cycles; and present a δ13C-stratigraphic framework for the Tasman sea geochemical record in context of the global benthic stack. We also document the occurrence of LLTM (both in open- and marginal marine setting), which constitutes its first evidence from the Pacific, and hence corroborates its globally widespread occurrence. Also, we find the occurrence of MECO at Site U1509, consistent with its expression in the Southern Ocean. A positive carbon and oxygen isotope excursion along with emergence of pure carbonate system is documented in New Caledonia within NP16 biozone of middle Eocene; however, its global relevance remains elusive because of lack of reliable age datum. Ultimately this study provides evidence for a dynamic climate in middle Eocene and shows that marine sedimentary carbon isotopes constitute a robust and reliable tool for Cenozoic chemostratigraphic correlation.