INFLUENCE OF NORTH ATLANTIC DEEP WATER on GLOBAL CARBON CYCLING DURING THE LATE MIOCENE

The onset of Northern Hemisphere glaciation, changes in continental vegetation, and a modest but permanent decline in the isotopic composition of the global carbon reservoir indicate that Earth surface processes changed markedly during the late Miocene. Here we investigate the role of North Atlantic Deep Water (NADW) circulation in global carbon cycling during the late Miocene. Study of deep-sea records recovered from a depth transect drilled along the flank of Walvis Ridge reveals that patterns of carbonate sedimentation changed in the southeastern Atlantic Ocean during the early-late Miocene (ca. 10 Ma). The shift in deep-sea sedimentation toward patterns of more modern aspect entailed a change from pelagic clay to calcareous ooze at two sites (ODP Sites 1262 and 1266) separated by ~1 km water depth. In general, wt.% CaCO₃ and wt.% coarse-fraction contents increase at both sites from ≤ 2% to ~85% and from < 5% to ~20%, respectively. This change in carbonate sedimentation reflects a deepening of the calcite compensation depth (CCD) in the Angola Basin where it dropped from ~3500 m to below 4500 m. Deepening of the CCD in the southeastern Atlantic is coincident with shoaling of the CCD in the eastern Pacific; hence, this change was likely caused by the tectonic emplacement of a bathymetric sill in the Central American Seaway region that rerouted relatively young NADW toward the South Atlantic. Moreover, a macrostratigraphy dataset consisting of 242 gap-bound sediment packages compiled from 74 DSDP-ODP-IODP sites reveals that the total number of carbonate packages preserved in the Atlantic Ocean basin increased sharply beginning at ~8 Ma. Preliminary benthic foraminiferal δ¹³C records for the deep- (Site 1262) and intermediate-water (Site 1266) sites indicate that δ¹³C_DIC decreased by ~1‰ at 10 Ma and 8 Ma, respectively. Depth-dependency in the timing of this δ¹³C signal is puzzling; still, we postulate that this isotopic decrease was driven by enhanced burial of ¹³C-rich inorganic carbon relative to ¹³C-depleted organic carbon in seafloor sediments. Our findings underscore the influence of tectonic controls on ocean circulation and thus global climate, and indicate that NADW production/routing may explain many of the changes observed in carbon cycling and the climate system during the late Miocene.