Abstract:
Spatial patterns of deep-sea carbonate sedimentation changed globally during the middle-to-late Miocene transition (12-7 Ma). This shift in pelagic sedimentation is signaled by intensified carbonate dissolution and shoaling of the calcite compensation depth (CCD) in the eastern Pacific and Caribbean regions, an event coined the "carbonate crash". Previous studies have attributed the carbonate crash to the southerly rerouting of Northern Component Water (NCW) via the emplacement of a bathymetric sill during the early stages of the Central American Seaway closure. Here we examine how carbonate saturation of abyssal waters in the southeastern Atlantic changed in response to NCW rerouting by reconstructing patterns of carbonate sedimentation in the Angola Basin. Carbonate mass accumulation rate (MAR) records were generated for four sites arrayed along a 2.2 km depth transect running up the northern flank of the Walvis Ridge. In descending bathymetric order the sites are: Site 1264 (2505 m), intermediate Site 1266 (3798 m), Site 1267 (4355 m), and Site 1262 (4755 m). The study sections were correlated using shipboard (ODP Leg 208) litho- and biostratigraphic data with each record chronicling the past 18 Ma of sedimentation. In contrast to the carbonate crash in other regions, our records show that this transition is marked by a shift from pelagic clay to calcareous ooze at the two deepest sites beginning at 12 Ma. This lithostratigraphic change reflects the gradual descent of the CCD by about 300 meters. Suppression of the CCD is consistent with an increased influx of relatively young NCW in the Angola Basin; however, it also coincides with a 3-fold increase in carbonate MARs at shallow Site 1264. This latter finding reflects deepening of the lysocline likely driven by increased surface-ocean carbonate production. The general trend delineated by the Site 1264 carbonate MAR record is congruous with the "biogenic bloom" hypothesis invoking elevated surface-ocean productivity as a driver for the spatial changes in deep-sea sedimentation marking the middle-to-late Miocene transition. The changes in deep-sea sedimentation herein reported represent an important step in the development of modern-like ocean circulation, which likely played a prominent role in reducing atmospheric greenhouse gas levels at this time.