Deep‐Water Carbonate Ion Concentrations in the Western Tropical Pacific Since the Mid‐Pleistocene: A Major Perturbation During the Mid‐Brunhes

Abstract

AbstractWe present a new deep‐water carbonate ion concentration ([CO32−]) record, reconstructed from the “size‐normalized weight” of the planktonic foraminifer Neogloboquadrina dutertrei in core MD06‐3047B, representing a middepth site (2.5 km) in the western tropical Pacific since 700 ka. On glacial‐interglacial time scales, deep‐water [CO32−] exhibits an inverse relationship with global sea‐level elevations, consistent with the “coral reef hypothesis” that the deep Pacific carbonate system responded to variations in shelf‐carbonate production through the past 700 kyr. On longer time scales, a decoupling between deep‐water [CO32−] and δ13C around the globe can be explained by a combination of continental weathering and nutrient inputs. During the mid‐Brunhes interval (~600–200 ka), [CO32−] reached a maximum of ~100 μmol kg−1 at the marine isotope stage (MIS) 12/11 boundary, followed by a steep decrease to a minimum of ~40 μmol kg−1 during middle MIS 11, representing the largest‐amplitude change in [CO32−] over the past 700 kyr. The [CO32−] maximum records the largest deglacial oceanic carbon release since 700 ka, and the [CO32−] minimum was a response to a global increase in pelagic carbonate production. From MIS 3 to 2 and from early to middle MIS 13, [CO32−] showed rising trends opposite to those at water depths greater than 3.4 km, implying enhanced Pacific stratification during these intervals. These findings provide new insights into the Pleistocene evolution of the carbonate system in the Pacific Ocean.