Surface and Deep Water Processes were Closely Related during Abrupt Change in the North Atlantic over the Past 420000 Years
24 Aug, 2007 11:50 am
A recent study (Martrat et al., 2007) based on high-resolution analyses of proxies for sea surface temperature, water mass distribution and relative biomarker content in marine sediment cores from the Iberian Margin shows that centennial climate variability over the last ice age exhibited a clear bipolar behaviour over the last 420,000 years. Abrupt cold episodes also occurred after relatively warm and largely ice-free periods when the predominance changed from northern deep waters to southern.
Institute of Chemical and Environmental Research of Barcelona (IIQAB). Spanish Council for Scientific Research (CSIC).
The study published in Martrat et al. (Science 317, 502-507, 2007) shows that the abrupt climate changes that occurred in the past 420,000 years were simultaneous with intensified pulses of Antarctic bottom water arriving to latitudes as north as those of the Iberian margin.
These changes are variations that led to strong coolings which lasted about 1,000-3,000 years. After these cooling stages the abrupt changes ended with a rapid and strong warming returning to initial conditions. The strongest changes of this type occurred in the Northern hemisphere.
These abrupt changes respond to the natural climate evolution of the planet. They are not related to human influence. They are faster than the climate processes depending from orbital forcing that led to glacial and interglacial periods. These last processes result from the accumulated effect of insolation in periods of 21,000, 41,000 and 100,000 years. The abrupt changes occurred in much shorter time intervals and their origin is still unknown. As consequence of these changes, the vegetation over the Iberian Peninsula, for example, was modified changing from the present type to steppe in time intervals of about 100 years or less.
The abrupt changes are specifically relevant at human scale because they encompass climate variations in time scales short enough as to be noticed in the time frame of a regular human life. The intensities of many of these changes were similar to those involving the transitions between glacial and interglacial periods. Although these changes were not stable over large time periods they lasted over 1,000-3,000 years, which is long enough to constitute long stages on a human perspective.
The work done shows that abrupt changes occurred both in glacial and interglacial periods. In the first case they were more frequent but their intensity was higher in the second.
Despite abrupt changes are defined by observation of the climate conditions occurring in the ocean surface and continental land masses, our work shows that they occurred simultaneously to reorganization of the deep water masses in the north Atlantic Ocean. Specifically they were concurrent with arrival of Antarctic deep waters at latitudes such as those of the Iberian Peninsula. In many cases it is observed that the arrival of Antarctic bottom waters preceded the abrupt changes. These results reveal that the climate processes that occurred in Antarctica were very relevant for the development of these abrupt changes in the northern hemisphere. The observed link is surprising having in mind that the abrupt changes were of higher intensity in the northern than the southern hemispheres.
This study is based on sediment cores recovered in the Atlantic Ocean near the Iberian Peninsula. In this site the influence of both Arctic and Antarctic waters can be observed.
To develop this study new biomarkers for description of the past oceanographic conditions have been developed. One of those records the intensity of the bottom ocean currents by measuring the degree of oxic transformation of mixtures of higher plant alkanes and alcohols. This marker shows an excellent correspondence with indicators of arrival of Antarctic or arctic waters such ä13C in benthic foraminifera. On the other hand, abrupt changes along the last 420,000 years have been identified from the study of the ratio between di- and triunsaturated alkenones that records sea surface temperature.
Martrat B., et al, Four Climate Cycles of Recurring Deep and Surface Water Destabilizations on the Iberian Margin, Science, Science 27 July 2007: Vol. 317. no. 5837, pp. 502 - 507