Multi-Proxy Approach to Terrestrial Input and Shifts in Circulation in the Eastern Equatorial Pacific from LGM to Present: Foraminiferal Mn/Ca, Sr/Ca and Mg/Ca

Earlier this year we published a paper introducing the Mn/Ca ratio of foraminifera as a new proxy (Geochem. Geophys. Geosyst., 10, doi:10.1029/2008GC002219). This paper examined Mn/Ca and other proxies for the last 30 ky in foraminifera from the Cocos Ridge at ODP Site 1242 in the extreme Eastern Tropical North Pacific (7° 51.35'N, 83° 36.42'W). We found that the Mn/Ca ratios in tests of the planktonic foram G. ruber indicated anomalously high input of dissolved terrestrial to the surface ocean during deglaciation. This input decreased from 15 ky to present and also back through the LGM, where we found the lowest ratios. We measured a smaller anomaly in the benthic foram Uvigerina perigrina at the same site and found that the bottom anomaly occurred after the one at the surface. This pattern raises a number of questions related to the source of the terrigenous maxima associated with Termination 1, extent of the anomalies, and their propagation through the oceans. As a first step toward addressing these questions, this paper presents comparable metal ratio data for Site 1238 in the EEP near the equator (1°52.31'S, 82°46.93'W; 2003m), about 200 km off the coast of Ecuador. The surface Mn/Ca anomaly at Site 1238 is remarkably similar in age and size to that at Site 1242. The anomaly in the benthics is again smaller than the one in the planktonic foraminifera being consistent with the vertical distribution of Mn in the oceans. Bottom Water Temperature at Site 1238 estimated from Mg/Ca was 2°C lower in the LGM. The benthic Mn/Ca anomaly follows δO18 more closely than BWT during the LGM-Holocene transition, consistent with the advective nature of this signal. The core of the bottom water anomaly at Site 1238 is characterized by a 10% negative anomaly in Sr/Ca consistent with the freshening of Antarctic Bottom Water at T1. This comparative study suggests that foraminiferal Mn/Ca ratios from the surface are not predominately related to dust input but associated with runoff and circulation as related to advection and the depth of the mixed layer. The benthic anomaly must be advective and in some situations may prove to be a reliable indicator for compositional changes in major water types.