Upper North Atlantic Deep Water variability during the Holocene
T. Marchitto, P. deMenocal
Keynote talk presented at 2002 Goldschmidt Conference
Abstract. The earth’s climate has been relatively stable over the past 10,000 yr (the Holocene epoch) when compared with glacial times. However, recent work in the North Atlantic has established the presence of significant Holocene cooling events that recur approximately every 1-2 kyr. These events are evident in the southward extent of drift ice and in inferred sea surface temperatures, but as of yet there has been no convincing chemical evidence for associated changes in deep water circulation. Here we examine the Holocene properties of upper North Atlantic Deep Water (NADW) using high-resolution sediment cores from two sites in the North Atlantic.
Ventilation of Labrador Sea Water (LSW), the uppermost component of NADW, is known to be very sensitive to surface ocean forcing today. We present benthic foraminiferal (Cibicidoides pachyderma) Mg/Ca and d18O data from a core on the Laurentian Slope (~1800 m depth) that is well situated to monitor the properties of LSW during the late Holocene. Mg/Ca indicates that during cold periods such as the Little Ice Age, LSW was colder by ~1-3C. Paired d18O measurements suggest that cooling was accompanied by freshening, resulting in lower densities during cold periods. This coupling between temperature and salinity is also characteristic of the LSW historical record, but with a much smaller range of variability than we observe on millennial timescales.
Farther downstream, core ODP 658 off northwest Africa (~2300 m depth) is bathed by NADW with a slight admixture of high-nutrient Circumpolar Deep Water. Since Cd and Zn behave like nutrients in seawater, benthic foraminiferal records of these two trace metals may be used to reconstruct the competition between northern and southern source deep waters during the past. Preliminary benthic (C. wuellerstorfi) Cd/Ca and Zn/Ca results do not exhibit any obvious relation to the 1-2 kyr Holocene cooling events. Cd/Ca and Zn/Ca do, however, suggest a substantial long-term decrease in nutrients since the mid Holocene. This could indicate an increase in the production of NADW associated with late Holocene (Neoglacial) cooling. Alternatively, it could represent an evolution of “preformed” nutrient concentrations in the North Atlantic.
Back to Current Projects
Back to Tom Marchitto Main Page