Middle to Late Miocene Benthic Foraminiferal Mg/Ca, a new Tracer for NADW?
Abstract
Changes in the flux of North Atlantic Deep Water (NADW) have been invoked as a causal mechanism for Quaternary millennial scale variations. The "Atlantic Salt Oscillator" is thought to respond to salinity changes in the North Atlantic caused by input of meltwater from northern hemisphere ice-sheets and by the balance between evaporation and inter-basin exchange of water masses. It has also been proposed that a "greenhouse effect" may lower the density of North Atlantic surface waters through increased temperatures and hydrologic intensity. The Miocene provides the best opportunity for studying the causes and consequences of changes in NADW in a relatively warm climate. Benthic foraminiferal Mg/Ca has previously been shown to have potential as a paleotemperature proxy on Cenozoic timescales (Lear et al., 2000). A new multispecies record has been constructed from ODP Site 926, presently situated at 3600 m water depth on the Ceara Rise in the equatorial Atlantic. This record covers the interval 4 to14 Ma with a resolution of about 200 ka. The most prominent feature of the record is a Mg/Ca increase between 7 and 11 Ma which is interpreted as an increase in bottom water temperatures of 3 to 4 degrees. The onset of the warming at Ceara Rise corresponds to increased carbonate preservation in the Atlantic (ODP Leg 154 sediments), and increased carbonate dissolution in the Caribbean (ODP Leg 165 sediments). The Mg/Ca signal is interpreted as representing an increase in the flux of NADW starting at 11 Ma. Comparison of the Mg-temperature record with the Ceara Rise benthic foraminiferal δ 18O record enables variations in salinity to be estimated. The data indicate that higher Mg-temperatures correlate with more saline waters at Ceara Rise, and that large salinity contrasts existed between different bottom water sources at this time.
- Publication:
-
AGU Spring Meeting Abstracts
- Pub Date:
- May 2001
- Bibcode:
- 2001AGUSM..OS32A09L
- Keywords:
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- 1050 Marine geochemistry (4835;
- 4850);
- 1065 Trace elements (3670);
- 1635 Oceans (4203);
- 9325 Atlantic Ocean;
- 9604 Cenozoic