Upper water column hydrology changes off Cape Hatteras and Gulf Stream activity over the Holocene

Modern oceanographic studies show that most of the ocean heat content in the North Atlantic Western Boundary Current region is stored in the upper 400 meters. To constrain past ocean heat content is important to understand past climate changes as this parameter controls heat flux to the atmosphere. We developed proxies for the upper water column hydrology in this area and used them to reconstruct the changes, over the Holocene, of the upper water column temperature and salinity off the Cape Hatteras. Deep-dwelling planktonic foraminifera living at different water depths may provide a proxy for upper water column hydrological changes. Calcification depths of Globorotalia inflata, Globorotalia truncatulinoides and Pulleniatina obliquiloculata have been constrained by correlating modern hydrographic data to oxygen isotopic measurement of late Holocene North Atlantic core-top samples. We found that the three deep-dwelling foraminifera species have a preferred habitat at the base of the seasonal thermocline (Cleroux et al, 2007). The same set of North Atlantic core-tops has been used to define relationships between trace elemental compositions and temperature. Relationships found for Globigerina bulloides and Globigerinoides ruber agree with previous calibrations for these species. We established calibrations between Mg/Ca ratio and temperature for the three deep-dwelling foraminifera species and between Sr/Ca ratio and temperature for G. inflata and G. truncatulinoides (Cleroux et al, submitted). To study Holocene heat content and Gulf Stream activity, we performed coupled analyses of oxygen isotopic and trace elemental composition on several foraminifera species from core MD99-2203 located off Cape Hatteras where the Gulf Stream separates from the United States coast. This location is the meeting point of three water masses: the southward flowing Labrador current with fresh and cold water; the warm Gulf Stream water circulating toward north-east and the deep-mixed layer Mode Water (18° C water) originating from the Sargasso Sea. Under modern condition, the core site is at the western limit of the Mode Water extension. We analysed G. ruber to constrain past surface water condition, P. obliquiloculata and G. truncatulinoides dextral to reconstruct seasonal and main thermocline conditions. High-resolution surface reconstructions over the Holocene show low amplitude periodic temperature and salinity changes that could be related to NAO type mechanisms. Large hydrological changes in sub-surface reflect variations of Labrador current and Mode Water influences. We conclude that the Gulf Stream activity was low from 14 to 8 ka off Cape Hatteras, with a strong Labrador current and mode water system. From 8 to 4 ka, the Gulf Stream activity gets stronger decreasing the contribution of the Labrador and mode waters. The modern situation was established around 4 ka.