Deep ocean storage of heat and CO2 in the Fram Strait, Arctic Ocean during the last glacial period

MME is funded by the Research Council of Norway and the Co-funding of Regional, National, and International Programmes (COFUND) Marie Sklodowska-Curie Actions under the EU Seventh Framework Programme (FP7), project number 274429, and the Research Council of Norway through its Centres of Excellence funding scheme, grant number 223259.The Fram Strait is the only deep gateway between the Arctic Ocean and the Nordic Seas and thus is a key area to study past changes in ocean circulation and the marine carbon cycle. Here, we study deep ocean temperature, δ18O, carbonate chemistry (i.e., carbonate ion concentration, [CO32-]), and nutrient content in the Fram Strait during the late glacial (35,000-19,000 years BP) and the Holocene based on benthic foraminiferal geochemistry and carbon cycle modelling. Our results indicate a thickening of Atlantic water penetrating into the northern Nordic Seas, forming a subsurface Atlantic intermediate water layer reaching to at least ~2600 m water depth during most of the late glacial period. The recirculating Atlantic layer was characterized by relatively high [CO32-] and low δ13C during the late glacial, and provides evidence for a Nordic Seas source to the glacial North Atlantic intermediate water flowing at 2000-3000 m water depth, most likely via the Denmark Strait. In addition, we discuss evidence for enhanced terrestrial carbon input to the Nordic Seas at ~23.5 ka. Comparing our δ13C and qualitative [CO32-] records with results of carbon cycle box modelling suggests that the total terrestrial CO2 release during this carbon input event was low, slow, or directly to the atmosphere.Publisher PDFPeer reviewe

Fast and slow components of interstadial warming in the North Atlantic during the last glacial

The abrupt nature of warming events recorded in Greenland ice-cores during the last glacial has generated much debate over their underlying mechanisms. Here, we present joint marine and terrestrial analyses from the Portuguese Margin, showing a succession of cold stadials and warm interstadials over the interval 35–57 ka. Heinrich stadials 4 and 5 contain considerable structure, with a short transitional phase leading to an interval of maximum cooling and aridity, followed by slowly increasing sea-surface temperatures and moisture availability. A climate model experiment reproduces the changes in western Iberia during the final part of Heinrich stadial 4 as a result of the gradual recovery of the Atlantic meridional overturning circulation. What emerges is that Greenland ice-core records do not provide a unique template for warming events, which involved the operation of both fast and slow components of the coupled atmosphere–ocean–sea-ice system, producing adjustments over a range of timescales

Ventilation history of Nordic Seas overflows during the last (de)glacial period revealed by species-specific benthic foraminiferal 14 C dates

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 Faroe Marine Ash Zone II, dated to ~26.7 ka, enables us to determine that the middepth (1179 m water depth) and shallow subsurface reservoir ages were ~1500 and 1100 14C 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 14C ventilation ages that are derived from dating different benthic foraminiferal species: Pyrgo and other miliolid species yield ventilation ages >6000 14C years, while all other species reveal ventilation ages <2000 14C years. These data either suggest subcentennial, regional, circulation changes or that miliolid-based 14C 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 interstadial (14.5 ka) is clearly marked by an increase in middepth Nordic Seas ventilation and the renewal of a stronger overflow

Fast and slow components of interstadial warming in the North Atlantic during the last glacial

Abstract: The abrupt nature of warming events recorded in Greenland ice-cores during the last glacial has generated much debate over their underlying mechanisms. Here, we present joint marine and terrestrial analyses from the Portuguese Margin, showing a succession of cold stadials and warm interstadials over the interval 35–57 ka. Heinrich stadials 4 and 5 contain considerable structure, with a short transitional phase leading to an interval of maximum cooling and aridity, followed by slowly increasing sea-surface temperatures and moisture availability. A climate model experiment reproduces the changes in western Iberia during the final part of Heinrich stadial 4 as a result of the gradual recovery of the Atlantic meridional overturning circulation. What emerges is that Greenland ice-core records do not provide a unique template for warming events, which involved the operation of both fast and slow components of the coupled atmosphere–ocean–sea-ice system, producing adjustments over a range of timescales

North Atlantic–Norwegian Sea exchanges during the past 135,000 years: Evidence from foraminiferal ∆14C, d11B, d18O, d13C, Mg/Ca, and Cd/Ca

The ocean is an essential regulator of climate through its effect on redistribution of heat and air-sea exchange of greenhouse gases.The modern circulation pattern of surface and deep water between the Arctic Mediterranean seas and North Atlantic is an important part of the global meridional ocean circulation. In this study we investigate the North Atlantic-Norwegian Sea exchanges of heat and carbon in connection to the glacial and deglacial climatic anomalies and carbon cycle evolution. During the last glacial and deglacial periods, abrupt and large amplitude switches in the northern hemisphere climate, particularly over Greenland, from cold (stadials) to warmer conditions (interstadials) were recorded. On the basis of isotopic and elemental composition of the fossil shells of planktic and benthic foraminifera, we assess the evolution in the seawater temperature, salinity, pH, CO2 concentration, nutrient levels and 14C ventilation ages from the southern Norwegian Sea. Our results show that the Norwegian Sea hydrography and it exchange with the North Atlantic changed in pace with past changes in regional climate, suggesting an important role. For instance, we show that the intermediate water in the Norwegian Sea has warmed-up by 2-5 °C during the cold stadials relative to the Holocene and interstadials. This subsurface build-up of heat during the cold stadials may have destabilized the water column, melted sea ice and ice shelves and thus significantly contributed to the onset of interstadial conditions. Our data show that glacial pH was elevated by ~0.2 units in the shallow subsurface Norwegian Sea compared to the Holocene. Brief episodes of acidification during some Heinrich stadials were recorded. Our results of intermediate water ventilation enabled us to evaluate and partly reject a suggestive hypothesis that the Nordic seas may have been a pathway for the mid-depth deglacial ∆14C anomalies recorded in North Atlantic

Deep Ocean storage of heat and CO2 in the Fram Strait, the Arctic Ocean during the last glacial period

Mg/Ca and B/Ca, δ18O, δ13C, Bottom Water Temperature (BWT), Seawater δ18O based on the benthic foraminiferal species Oridorsalis umbonatus from sediment core core HH12-946MC, the Fram Strait

Millennial‐scale changes in bottom water temperature and water mass exchange through the Fram Strait 79ºN, 63–13 ka

The Svalbard margin, in the eastern Fram Strait with its high sediment accumulation, form a key area for the reconstruction of water mass and heat exchange between the North Atlantic and Arctic Ocean in relation to abrupt climate changes as seen in glacial Greenland Interstadial and Greenland Stadial (GI‐GS) events. Here, we present a bottom water temperature (BWT) record from the northern Nordic Seas (79°N) at 1273 m water depth based on benthic foraminiferal Mg/Ca. The BWT reconstructions, combined with benthic foraminiferal stable isotopes, benthic foraminiferal fauna compositions and ice rafted debris (IRD), reveal at least two distinctive scenarios for the GI‐GS events during the last glacial period (13–63 ka). During GIs, conditions were similar to modern with high productivity, low BWT and deep convection. During GS6, GS8 and GS15 and during Heinrich Stadials (HSs), BWT increased up to 5±1°C generally concomitant with low planktic and benthic δ18O. Our results suggest, that during some GSs and HSs, deep water generation was reduced, allowing the subsurface Atlantic water to thicken and deepen down to at least the core site depth. A strong halocline during HSs and GSs prevented heat release from the subsurface Atlantic water, which we can now trace from 45ºN in the North Atlantic to the Arctic Ocean >79ºN. Surfacing of the salty Atlantic subsurface water pre‐conditioned the Nordic seas for convection. Release of the subsurface heat from this vast reservoir must have contributed to the large and abrupt atmospheric warmings at the start of Greenland interstadials