ALBERT

Description: Formation of deep-water in the high-latitude North Atlantic is important for the global meridional ocean circulation and its variability in the past may have played an important role in regional and global climate change. Here, we study ocean circulation associated with the last (de)glacial period, using water-column radiocarbon age reconstructions in the Faroe-Shetland Channel, southeastern Norwegian Sea and from the Iceland Basin, central North Atlantic. The presence of tephra layer FMAZ II, dated to ~26.7 ka, enables us to determine that the mid-depth (1179 m water depth) and shallow subsurface reservoir ages were ~1500 and 1100 14 C years, respectively older during the late glacial period compared to modern, suggesting substantial suppression of the overturning circulation in the Nordic Seas. During the late Last Glacial Maximum and the onset of deglaciation (~20–18 ka), Nordic Seas overflow was weak but active. During the early deglaciation (~17.5–14.5 ka), our data reveal large differences between 14 C ventilation ages that are derived from dating different benthic foraminiferal species: Pyrgo and other miliolid species yield ventilation ages 〉6000 14 C years, while all other species reveal ventilation ages 〈2000 14 C years. These data either suggest major short-term circulation changes that have been hitherto ignored in palaoeceanographic studies or that miliolid-based 14 C ages are biased due to taphonomic or vital processes. Implications of each interpretation are discussed. Regardless of this ‘enigma’, the onset of the Bølling-Allerød interstadials (14.5 ka), is clearly marked by an increase in Nordic Seas ventilation and the renewal of a strong overflow.

Description: [1]  Antarctic Intermediate Water (AAIW) and Subantarctic Mode Water (SAMW) are the main conduits for the supply of dissolved silicon (silicic acid) from the deep Southern Ocean to the low latitude surface ocean, and therefore have an important control on low latitude diatom productivity. Enhanced supply of silicic acid by AAIW (and SAMW) during glacial periods may have enabled tropical diatoms to outcompete carbonate-producing phytoplankton, decreasing the relative export of inorganic to organic carbon to the deep ocean and lowering atmospheric p CO 2 . This mechanism is known as the ‘Silicic Acid Leakage Hypothesis’ (SALH). Here we present records of neodymium and silicon isotopes from the western tropical Atlantic that provide the first direct evidence of increased silicic acid leakage from the Southern Ocean to the tropical Atlantic within AAIW during glacial Marine Isotope Stage (MIS) 4 (~60–70 ka). This leakage was approximately coeval with enhanced diatom export in the NW Atlantic and across the eastern equatorial Atlantic and provides support for the SALH as a contributor to CO 2 drawdown during full glacial development.

Description: Knowledge of past deep-ocean current speeds has the potential to inform our understanding of changes in the climate system on glacial-interglacial timescales, because they may be used to help constrain changes in deep-ocean circulation rates and pathways. Of particular interest is the paleo-flow speed of southern-sourced deep water, which may have acted as a carbon store during the last glacial period. A location of importance in the northward transport of southern-sourced bottom water is the Vema Channel, which divides the Argentine and Brazil basins in the South Atlantic. We revisit previous studies of paleo-flow in Vema Channel using updated techniques in grain size analysis (i.e., mean sortable silt grain size), in Vema Channel cores and cores from the Brazil margin. Furthermore, we update the interpretation of the previous grain size studies in the light of many years further research into the glacial circulation of the deep Atlantic. Our results are broadly consistent with the existing data and suggest that during the last glacial period there was slightly more vigorous intermediate to middepth (shallower than 2,600 m) circulation in the South Atlantic Ocean than during the Holocene, whereas around 3,500 m the circulation was generally more sluggish. Increased glacial flow speed on the eastern side of the Vema Channel was likely related to an increase in northward velocity of AABW in the channel. An increase in Antarctic Bottom Water flow through the Vema Channel may have helped to sustain the large volume of southern-sourced deep water in the Atlantic during the glacial period.