Records of the Nd isotope composition of seawater from the Bay of Bengal: Implications for the impact of Northern Hemisphere cooling on ITCZ movement

Abstract

This study presents a record of planktonic foraminiferal neodymium isotopic gradients along a north–south transect in the Bay of Bengal during time slices of late Holocene and last glacial maximum (LGM) age, together with a record of planktonic foraminiferal variation in the northern Bay of Bengal (15°N) over the last 195 ky. In late Holocene core top planktonic foraminifera, the north–south ε Nd gradient rises from nonradiogenic values of − 12 at 20°N to − 10 at 5°N, in parallel with the modern surface salinity gradient controlled by discharge of Himalayan rivers in the northern Bay. During the LGM, ε Nd increased throughout the Bay, the contrast between northernmost and southernmost sites decreased, and maximum ε Nd values of − 6.5 occurred between 12 and 15°N. A small part of the shift to higher mean ε Nd throughout the Bay during the glacial may arise from a uniform increase in deposition of far-field dust from Arabian and Persian Gulf regions. However, the spatial pattern of ε Nd variation between LGM and late Holocene also suggests a shift from modern dominance of nonradiogenic Nd sources from the Ganges–Brahmaputra basin to LGM dominance of more radiogenic Nd sources from Arakan coastal rivers. Over the last 195 ky at 15°N, the most radiogenic ε Nd values of − 7 occur at glacial maxima and the most nonradiogenic values of − 11 occur during interglacials. ε Nd values are highly correlated with glacial interglacial variations in planktonic foraminiferal δ 18O. In this record, shifts in river sources from the more northerly Ganges–Brahmaputra watershed to the more southerly Arakan coastal river systems respond dominantly to Inter-Tropical Convergence Zone (ITCZ) movement driven by Northern Hemisphere cooling during 100 ky glacial/interglacial cycles, with a small component of variation on precessional timescales. The nonlinear correlation of ε Nd with ice volume suggests that ITCZ movement responds to aerial coverage of ice sheets and snow rather than to ice thickness and volume. These data add support to recent general circulation models of ITCZ response to Northern Hemisphere ice sheets, which simulate decreased glacial precipitation in the Ganges–Brahmaputra basin and increased glacial precipitation in the southern Arakan coastal basin.