Do periplatformal slopes and basins in the Bahamas preserve shallow carbonate geochemistry?

Abstract:

Shallow carbonates represent one of the most abundant archives of Earth history. Specifically, carbonate facies, mineralogy, and isotope geochemistry serve as proxies for paleoclimate parameters such as the global carbon cycle and relative sea level. Understanding the mechanisms controlling the mineralogy and geochemistry of modern carbonates therefore is key to accurate reconstruction of past climates. The ancient record principally preserves shallow carbonate, but we lack modern analogs to these pre-Cenozoic shallow water stratigraphies. Platform records older than a few thousand years were overprinted by meteoric diagenesis during the last glacial sea level lowstand, so we instead must look to the periplatformal slope and proximal basins for a modern analog of platform production and geochemistry. Shallow carbonate is exported to deeper waters from the shelf edge, where it is protected from erosion and meteoric diagenesis during glacial sea level lowstands. Periplatform sediments thus contain a record of shelf-derived carbonate that is continuous across ice ages. Using 21 short piston cores from around Bahamian platforms, we integrate measurements of δ¹³C, δ¹⁸O, and mineralogy with quantitative petrographic observations to understand what fraction of carbonate is produced in the mixed layer, what fraction is from the shallow shelf, and how both are altered by early marine diagenesis at the sea floor. By deconvolving these signals, we construct a record of shallow carbonate production/export, δ¹³C, and diagenesis over the past 150 kyr. We use this record to understand how shallow environments respond to sea level oscillations during the most recent Pleistocene glacial cycles. Our study of recent periplatform carbonates in the Bahamas can inform interpretations of carbonate facies and δ¹³C in ancient periplatformal records.