Glacial-interglacial sedimentary processes in the Northeast Atlantic Ocean as reveal grain-size analyses

Abstract

High resolution grain-size results of core KTA-GC-05 are presented in order to infer millennial and submillennial changes of open ocean transport processes. The core was collected using a gravity corer on board the B/O Sarmiento de Gamboa during PALEOACID cruise of CATARINA project in August 2012. It is located in the eastern basin of the North Atlantic Ocean at 46º 11.157¿N and 18º 18.837¿W, at a depth of 3939 meters. The core length reaches 337 cm and an expected age of around 120 ka. Grain-size analyses were performed on both the total fraction (organic matter removed with 10% H2O2) and the non-carbonate fraction (both organic matter and carbonate were removed with H2O2 and HCl, respectively). Grain-size distributions were measured with a Beckman Coulter LS 13 320 laser particle size analyser (CLS), which determines particle grain-sizes between 0.04 and 2000 m as volume percentages based on diffraction laws (McCave et al., 1986; Agrawal et al., 1991). Results for every sample include the median (D50), the mean and the first mode. Also percentages of sand, silt and clay fractions are shown. In addition, the UP10 size (i.e. particles coarser than 10 m) is considered, which adds the fine sand subpopulation to the sortable silt size fraction (SS, 10-63 m). The sortable silt size (SS) has been broadly used as a proxy to infer the intensity of deep water currents (McCave et al., 1995). However, since strong currents are also able to rework particles coarser than 63 m, the UP10 fraction has been also considered for the study of paleocurrent intensity (Frigola et al., 2008). Median and mean values of bulk and non-carbonated sediment fractions show different features thus pointing to different processes controlling the deposition of both fractions. Nevertheless, the noncarbonate fraction better represents the intensity of bottom currents (McCave et al., 1995) or the influence of iceberg discharge. Clear differences of sedimentological data between the Last Glaciation (MIS 2-4) and the Last Interglacial (MIS 5) are observed, suggesting much contrasted sedimentation patterns during both periods. During glacial times sedimentation is controlled by iceberg input, thus grain-size data does not reflect bottom current intensity. Besides, Atlantic Meridional Overturning Circulation (AMOC) was reduced during glaciations or even stopped during extreme cold events. Precisely, stronger sedimentological signals are recorded coinciding with these events (Heinrich Events, HE). This pattern is corroborated by other geochemical and micropaleontological proxies analysed in the same core. HEs are in general characterized by sharp increases of sand, D50, UP10 and silt/clay ratio of non-carbonate fraction as well as slight decrease of SS. Nevertheless, several differences between the younger HEs (HE1, HE2 and HE3) and the earlier HEs (HE4, HE5 and HE6) can be observed. On the other hand, during the Last Interglacial (MIS 5), with active AMOC observed fluctuations in the grain-size data are an accurate proxy for reconstructing bottom currents intensity. Variations in silt/clay ratio, UP10 and SS are in agreement with climate changes associated with MIS 5 substages. Warmer substages 5a, 5c and 5e are characterized by higher values of the above mentioned markers, pointing to more vigorous bottom circulation. In fact, lower percentages of clay fraction are recorded during these warmer intervals and coarse and very coarse silt are the dominant size from the non-carbonate fraction. The opposite trend occurs during relatively colder substages 5b and 5d, when percentages of clay and finer silt fractions increase, as well as SS and UP10 values decrease. These facts suggest sluggish bottom current