Multi-proxy evidence for the behaviour of the Atlantic Meridional Overturning circulation in the Neogene
Abstract
Though the overall pattern of the modern deep-water circulation in the Atlantic was probably established in the Neogene, there is also increasing evidence suggesting significant secular variation over the past 10-15 Ma forced by an array of potential processes. These include continued sill development in the North Atlantic, the advent of sea-ice in the Southern Ocean and the closure history of the Central American Seaway. Deciphering the relative importance of these different processes is not always straightforward using traditional circulation proxies, such as those based on deep-water nutrient distributions. For example, changes in the nutrient or 13C inventory of deep water in the Atlantic could track changes in one of three different phenomena: the pattern of mixing between southern- versus northern-derived water masses; the extent of nutrient-utilisation in the surface Southern Ocean; or vertical nutrient cycling at the location investigated. Only a multi-proxy approach to this problem can unpick these various controlling processes. Nd isotopes provide a relatively new and potentially powerful tracer of deep-water circulation in the Atlantic, and the deep water Nd isotope composition may be extracted at high time resolution from the authigenic fraction of deep-sea sediments. Such an approach has been shown to isolate circulation changes from those related to the carbon cycle and nutrient-cycling in the Late Quaternary1. We have already published2 combined records of foraminiferal Cd/Ca ratios and Nd isotopes for the Neogene of the deep South Atlantic (ODP Leg 208, site 1264). Here we present a higher resolution (up to 100 kyr) version of this dataset. In combination with studies of secular δ13C gradients3 down the deep Atlantic, these data help to clarify the processes that control deep water chemistry and isotope geochemistry. For example, the Nd isotope data eliminate ambiguities in the carbon data surrounding the relative importance of the pattern of deep water mass mixing (south versus north) versus changes in the source water masses themselves (e.g. sea-ice growth in the South or seaway history in the North). Together the data suggest NADW export to the South Atlantic at 3-4 Ma that is as strong as today. Prior to that Nd isotopes also imply relatively strong NADW export, while a relative lack of coherence between the nutrient-based proxies and Nd isotopes may record changes in the pre-formed nutrient characteristics of southern-derived water masses. 1Piotrowski, A.M. et al. (2005)Science 307, 1933-1938. 2Klevenz, V. et al. (2008) EPSL 265, 571-587. 3Hodell, D.A. and Venz-Curtis, K.A. (2006) Geochem. Geophys. Geosys. 7, Q09001, doi:10.1029/2005GC001211.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFMPP42B..01V
- Keywords:
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- 4912 Biogeochemical cycles;
- processes;
- and modeling (0412;
- 0414;
- 0793;
- 1615;
- 4805);
- 4924 Geochemical tracers;
- 4962 Thermohaline