A 370-ka paleoceanographic record from the Hess Rise, central North Pacific Ocean, and an indistinct ‘Kuroshio Extension’

Abstract

A 5 m long core (NP36) collected from the Hess Rise in the central North Pacific Ocean (34°11.7′N, 179°15.4′E, water depth 2664 m) was analyzed for benthic and planktonic foraminifera. This core yields a continuous record for the last ca. 370 000 years according to the oxygen isotope stratigraphy. Cross-spectral analysis of down-core variations in planktonic δ18O values correlates well with the SPECMAP reference scale allowing recognition of marine isotope stages (MIS) down to MIS 10 and the δ18O variations coincide well with the changes in sea surface temperature derived using the modern analogue technique. Time-series analyses of mass accumulation rates (MAR) of planktonic and benthic foraminifera were also performed along with analysis of variations in faunal composition. MAR of planktonic foraminifera display maxima during glacial periods which, together with coincident changes in lightness (L*) of sediment measured with the spectrophotometer, strongly suggests increasing flux of eolian dust to the Hess Rise due to strengthened westerly winds. However, MAR of benthic foraminifera including Epistominella exigua, a common typical phytodetritus feeder, also display highest increases during some interglacial periods (i.e. MIS 5 and 7). The benthic MAR maxima are thought to reflect variations in consumption patterns between benthic and planktonic species. A proposed hypothesis that vigorous grazing of phytoplanktons occurred during glacial periods reducing the flux of phytodetritus to the sea floor in turn impacting and limiting production of benthic foraminifera with the opposite effect prevailing during interglacial periods. Down-core variations in right-coiling Neogloboquadrina pachyderma abundance and the composition of planktonic foraminiferal faunas indicate the Subarctic Front did not shift southward over the study site during the past 370 000 years. Alternatively, variations in faunal composition indicate that Subtropical Water prevailed in the study area throughout this period. No clear faunal evidence of the presence of the ‘Kuroshio Extension’ was found in core NP36 reflecting the loss of heat and tropical faunal elements from this water mass as it flowed eastward during late Quaternary climatic maxima and minima.