Atlantic Meridional Overturning Circulation during Heinrich-Stadial 1 & 2 as seen by 231Pa/230Th

Abstract:

Assessing the sensitivity of the Atlantic Meridional Overturning Circulation (AMOC) is a major challenge for paleoclimatology, because its strength and structure is a crucial element of the global heat- and carbon distribution towards the deep ocean. Here the focus is set on how excessive freshwater input through abrupt melting of continental ice sheets can affect its overturning vigour. Such forcing can be tested by investigating its behaviour during extreme iceberg discharge events into the open North Atlantic during the last glacial period, so called Heinrich-Events [Heinrich 1988; Hemming 2004].

The sedimentary activity ratio ²³¹Pa/²³⁰Th has been increasingly used as a kinematic circulation proxy in the Atlantic Ocean over the past decade [Gherardi et al. 2009; McManus et al. 2004; Lippold et al. 2012]. Here we present ²³¹Pa/²³⁰Th ratios from several Atlantic sediment cores across Heinrich Events 1 (~17 ka BP) and 2 (~24 ka BP). The comparison of the profiles demonstrates the potential pitfalls when interpreating a single ²³¹Pa/²³⁰Th profile. E. g. core IODP 1313 (Mid Atlantic Ridge, 3412 m water depth) shows ²³¹Pa/²³⁰Th between 0.04 and 0.06, which would indicate a vigorous circulation over the entire time period. On the other hand core GeoB 16202-2 (Brasilan coastal area, 2248 m water depth) has a profile similar to the well known data set of [McManus et al. 2004] (i.e. during Heinrich Stadials values close to the production ratio of ~0.093, lower values at Holocene and LGM).

Such divergency can be explained by ²³¹Pa/²³⁰Th dependence on water depth, latitude, water mass and water mass age [Luo et al. 2010; Lippold et al. 2011], but also on changes in bioproductivity especially the flux of biogenic opal [Anderson et al. 1983A; Bradtmiller et al. 2007; Chase et al. 2002]. To avoid misleading interpretations, the here shown data set is accompanied by measurements of biogenic opal contents to appraise possible influences on the proxies. We observe large variations of opal contents - up to 10% at location IODP 1314 (close to Iceland, 2899 m water depth) in contrast to GeoB 16202-2 with an average of about 1-2% - and with a few data points showing correlation with ²³¹Pa/²³⁰Th.

With an inverse model approach [Luo et al. 2010] strength of the AMOC before, during and after the Heinrich-Stadials 1 & 2 will be derived from this database.