Atlantic Alkenone Sea-Surface Temperature Records

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Abstract

The variability of sea-surface temperatures (SST’s) in the Atlantic Ocean is examined for the past 370 kyr BP in order to unravel differences in climate change in low latitudes versus high latitudes for both hemispheres. For this purpose, special emphasis is given to variations in alkenone-derived SST estimates at time scales of 10 to 100kyr. Tropical SST records show relatively warm temperature values during the penultimate glacial marine oxygen isotope stage (MIS) 6, and further indicate that warming in the equatorial Atlantic began 10 to 15kyr prior to deglacial sea-level rise and melting of continental ice caps at the last three glacial terminations. The periodic glacial to interglacial changes in the temperature pattern closely resemble the 100 and 400kyr amplitude modulation of precessional insolation variations at low latitudes. This implies that, to a great extent, variations in surface-ocean temperatures are a direct response to low-latitude insolation forcing by eccentricity, and not just a result of eccentricity-related climate change in high-latitudes, e. g., changes in ice volume, thermohaline circulation and atmospheric CO2 concentration. Alkenone SST records from mid latitudes in the North and South Atlantic reveal SST patterns that are more congruent with marine oxygen-isotope records. Here, the coldest glacial SST’s occur simultaneously with maxima in ice volume at the end of glaciation periods rather than about 10 to 15kyr earlier as is observed in the tropical SST records. Nonetheless, the notably warm temperatures characteristic of the glacial MIS 6 in the tropics are also exhibited in SST curves off South Africa and in the North Atlantic. This indicates that part of the alkenone SST signal typical for the tropics is observable at mid latitudes, at sites influenced by surface currents originating in the tropical Atlantic or Indian Ocean. Comparison of alkenone SST records from low and mid latitudes therefore hints at a potential role for the tropics as an amplifier of the insolation forcing related to eccentricity, which may help to explain the strong response of global climate change to an orbital forcing relatively low in strength. Longer alkenone SST records are needed from tropical oceans to assure that the characteristic 100 and 400kyr periodicity in tropical SST variations is real for more than just the last three glacial-interglacial cycles.