Anomalous carbonate preservation in the abyssal North Pacific, ODP sites 882 and 1179: planktonic foraminiferal analysis, climate change and palaeogeomagnetism /

Abstract

Samples for this study were obtained from the abyssal western North Pacific Ocean at Ocean Drilling Project Sites 882 (50°21.797'N, 167°35.999^E) and 1179 (41°4.787^N, 159°58.786'E). Despite their depth below the present day calcite compensation depth, ~250m and ~ 1.5km respectively, these late Pliocene-Pleistocene cores contained large quantities of carbonate, a small proportion of which is composed of well-preserved foraminiferal tests with minor to no signs of dissolution. This excellent preservation is unusual in light of their positions below the current calcite compensation depth. Globigerina bulloides, Globigerina quinqueloba, and Neogloboquadrina pachyderma are the dominant species at the more northerly Site 882, whereas A^. pachyderma, Globorotalia inflata, and G. bulloides dominate Site 1179. This species distribution is a direct reflection of their respective ambient oceanographic conditions. Cluster analysis of the identified foraminifera reveals four groups within Site 882, which are largely temperature and dissolution-controlled, documenting a fluctuating calcite compensation depth and lysocline as well as indicating a switch to cooler conditions at about 2 Ma. The ODP fauna, although similar to those collected in nearby sediment traps, have a slightly lower diversity, and are enriched in thick walled, non-spinose taxa. Abundance peaks of foraminifera throughout the cores agree well with the timing of brief suppression of the calcite compensation depth. These suppressions of the calcite compensation depth are related to increased surface water productivity commensurate with terrestrial fertilization of otherwise nutrient-poor gyre areas. Furthermore, these events sequestered CO2 from the atmosphere, contributing to late Cenozoic global cooling. The majority of the peaks correlate with times of incursions of cold deep Pacific water and rapid declines of continental ice volume, suggesting that North Atlantic Deep Water interference may change the source dominance of Pacific deepwater, leading to less acidic, saltier water conditions, requiring less CaC03 to reach saturation. Several foraminiferal carbonate peaks also correlate with geomagnetic reversals inferring some climatic control, perhaps through enhanced cosmic ray bombardment and subsequent increased cloud cover, and species selection.