STABLE ISOTOPIC COMPOSITION OF SIZE FRACTIONS IN EARLY PALEOGENE SEDIMENTS:IMPLICATIONS ON BULK CARBONATE ISOTOPIC ANALYSIS

A series of short-term negative carbon isotope excursions (CIEs) characterize the early Paleogene, as perhaps best expressed in high-resolution bulk carbonate δ¹³C records. These CIEs (e.g., the PETM ca 56 Ma, ETM2 or H1 event ca ~54Ma, ETM3 or K/X event ca ~53 Ma) clearly mark changes in global carbon cycling and probably climate change, as at least some of them were associated with major warming events and deep-sea carbonate dissolution. The remarkable correlation of bulk carbonate δ¹³C records from different ocean basins during early Paleogene also raises an issue. This is because bulk carbonate contains multiple components, each of which might vary in abundance and isotope composition over space or time. To more fully address this perplexing issue, we examined 45 samples of pelagic sediments deposited between 57-51 Ma at ODP Site 1209 (Shatsky Rise, NW Pacific). Samples were examined for their bulk carbon and oxygen stable isotope compositions and then were separated into five different size fractions: <25 µm, which consisted of calcareous nannofossils, 25-63 µm, consisting of mainly foraminiferal fragments, 63-125 µm, and 125-250 µm, which consisted of both benthic and planktonic foraminifers and the largest size fraction >250 µm, consisting of large planktonic foraminifers, particularly mixed layer photosymbiont bearing Morozovellids. Each fraction was then weighed and analyzed for stable isotope composition. Difference between measured bulk carbon isotope and weighted sum of all size fractions and their isotope compositions (i.e. calculated bulk) vary up to ~0.25‰ on average. Our study uniquely addresses the multiple factors controlling this difference and analyzes what bulk carbon isotope composition implies. Another interesting finding of the study shows a significant increase in coarse fraction abundance at Site 1209 across PETM, which is suggested to be due to proliferation of mixed layer Morozovellid group of planktonic foraminifers in tropical oceans.