Abstract:
Records of ice-rafted detrital (IRD) particles and planktonic foraminifera provide evidence for massive, millennial-scale iceberg discharges, termed Heinrich events, into the glacial North Atlantic. These icebergs released large quantities of fresh water as they melted, significantly affecting the surface hydrography of this region. In this study, we use foraminiferal delta (super 18) O and Mg/Ca, which is a paleo-temperature proxy, to reconstruct the sea surface temperature (SST) and the seawater isotopic composition (delta (super 18) O (sub water) ) histories at DSDP site 609 in the eastern North Atlantic. We utilize two planktonic foraminifera, Neogloboquadrina pachyderma(sinistral) and Globigerina bulloides. Our records extend through the glacial-interglacial transition and the four most recent Heinrich events. Based on foraminiferal Mg/Ca. the glacial-interglacial SST change at DSDP site 609 is approximately 6 degrees C. This value is consistent with faunal estimates and Mg/Ca records from a nearby core, CHN82-4PC. The SST and delta (super 18) O (sub water) records also show major changes in surface hydrography associated with Heinrich events, which were generally accompanied by lower SST and reduced sea surface salinity. Of the three Heinrich events for which we have completed Mg/Ca and delta (super 18) O analyses. Heinrich 3 appears to have exerted the greatest effect on surface hydrography at DSDP site 609. This finding is somewhat puzzling, given the general perception that Heinrich 3 was associated with a relatively minor iceberg discharge and only a modest accumulation of IRD in the eastern North Atlantic. SST's based on Mg/Ca in N. pachyderma(s) correlate well with the % N. pachyderma(s) index at DSDP site 609. A linear correlation indicates that N. pachyderma(s) abundance reaches 0% at approximately 12 degrees C, which is consistent with other published temperature ranges for this species. The Mg/Ca-SST estimates do not demonstrate a strong correlation with the GISP2 delta (super 18) O (sub ice) record of air temperatures above Greenland. There are significant differences between the N. pachyderma(s) and G. bulloides data, suggesting that each species recorded different conditions. The observed offsets in delta (super 18) O and Mg/Ca between the species are consistent with the general view that N. pachyderma(s) lives in deeper, colder waters. Yet, of the two species, only N. pachyderma(s) seems to consistently record the hydrographic variations that accompanied the Heinrich events.