Northern Source Deep Water Research Articles

Spiraling pathways of global deep waters to the surface of the Southern Ocean

Upwelling of global deep waters to the sea surface in the Southern Ocean closes the global overturning circulation and is fundamentally important for oceanic uptake of carbon and heat, nutrient resupply for sustaining oceanic biological production, and the melt rate of ice shelves. However, the exact pathways and role of topography in Southern Ocean upwelling remain largely unknown. Here we show detailed upwelling pathways in three dimensions, using hydrographic observations and particle tracking in high-resolution models. The analysis reveals that the northern-sourced deep waters enter the Antarctic Circumpolar Current via southward flow along the boundaries of the three ocean basins, before spiraling southeastward and upward through the Antarctic Circumpolar Current. Upwelling is greatly enhanced at five major topographic features, associated with vigorous mesoscale eddy activity. Deep water reaches the upper ocean predominantly south of the Antarctic Circumpolar Current, with a spatially nonuniform distribution. The timescale for half of the deep water to upwell from 30° S to the mixed layer is ~60–90 years.

The role of deep ocean circulation in setting glacial climates

The glacial cycles of the Pleistocene involve changes in the circulation of the deep ocean in important ways. This review seeks to establish what were the robust patterns of deep‐sea water mass changes and how they might have influenced important parts of the last glacial cycle. After a brief review of how tracers in the modern ocean can be used to understand the distribution of water masses, I examine the data for biogeochemical, circulation rate, and conservative tracers during glacial climates. Some of the robust results from the literature of the last 30 years include: a shoaled version of northern source deep water in the Atlantic, expanded southern source water in the abyss and deep ocean, salt (rather than heat) stratification of the last glacial maximum (LGM) deep‐sea, and several lines of evidence for slower overturning circulation in the southern deep cell. We combine these observations into a new idea for how the ocean‐atmosphere system moves from interglacial to glacial periods across a single cycle. By virtue of its influence on the melting of land‐based ice around Antarctica, cooling North Atlantic Deep Water (NADW) leads to a cold and salty version of Antarctic Bottom Water (AABW). This previously underappreciated feedback can lead to a more stratified deep ocean that operates as a more effective carbon trap than the modern, helping to lower atmospheric CO2 and providing a mechanism for the deep ocean to synchronize the hemispheres in a positive feedback that drives the system to further cooling.

Sedimentary growth pattern on the northern Argentine slope: The impact of North Atlantic Deep Water on southern hemisphere slope architecture

Large sedimentary deposits consisting of several major contourite drifts were studied by means of high-resolution multichannel seismic data at the middle slope along the Northern Argentina Continental Margin to determine their evolutionary stages as well as to identify and assess the possible impact of Northern Source Deep Water (NSDW) on the slope architecture. The imaged contouritic sediments allow decoding on the regional paleo-oceanographic setting of the last 32Ma.Earliest contouritic sedimentation can be observed close to the Eocene/Oligocene boundary based on an aggradational stacking pattern with a complex and wavy seismic facies, pointing toward a hydrodynamically turbulent flow pattern. This facies is most likely related to the opening of the Drake Passage associated with global cooling and a strengthening of surface, intermediate and deep ocean currents in the Southern Ocean. During the Middle Miocene plastered drift sequences with an aggradational reflection pattern were deposited. Their depositional style indicates weak, non-turbulent current conditions, which are interpreted to be related to a vertical shift of water mass interfaces caused by the first formation of NSDW during the Mid-Miocene climatic optimum. On top, the formation of plastered drift sequences led to the modern extent of the Ewing Terrace, which was probably controlled by the continuous strengthening and thickening of NSDW until the final closure of the Central American Seaway (CAS). During the Pliocene and Quaternary, after the complete closure of the CAS and under the influence of the full force of the NSDW, mounded plastered drift sequences are built upon the Ewing Terrace generating the modern slope morphology. Therefore, we suggest that deep-water production in the northern hemisphere plays a significant role by controlling the shape of the continental slopes in the southwestern South Atlantic since the Middle Miocene.

The role of ocean cooling in setting glacial southern source bottom water salinity

At the Last Glacial Maximum (LGM), the salinity contrast between northern source deep water and southern source bottom water was reversed with respect to the contrast today. Additionally, Glacial Southern Source Bottom Water (GSSBW) was saltier than Antarctic Bottom Water (AABW), over and above the difference implied by the mean sea level change. This study examines to what extent cold temperatures, through their effect on ice formation and melting, could have caused these differences. Computational sensitivity experiments using a coupled ice shelf cavity–sea ice–ocean model are performed in a Weddell Sea domain, as a representative case study for bottom water formation originating from Antarctic continental shelves. Ocean temperatures at the domain open boundaries are systematically lowered to determine the sensitivity of Weddell Sea water mass properties to a range of cool ocean temperatures. The steady state salinities differ between experiments due to temperature‐induced responses of ice shelf and sea ice melting and freezing, evaporation and open boundary fluxes. The results of the experiments indicate that reduced ocean temperature can explain up to 30% of the salinity difference between GSSBW and AABW, primarily due to decreased ice shelf melting. The smallest and most exposed ice shelves, which abut narrow continental shelves, have the greatest sensitivity to the ocean temperature changes, suggesting that at the LGM there could have been a shift in geographical site dominance in bottom water formation. More sea ice is formed and exported in the cold ocean experiments, but the effect of this on salinity is negated by an equal magnitude reduction in evaporation.

Primary productivity variability on the Atlantic Iberian Margin over the last 70,000 years: Evidence from coccolithophores and fossil organic compounds

[1] This study analyzes coccolithophore abundance fluctuations (e.g., Emiliania huxleyi, Gephyrocapsa specimens, and Florisphaera profunda) in core MD01-2444 sediment strata retrieved at the Iberian Margin, northeastern Atlantic Ocean. Coccolithophores are calcareous nannofossils, a major component of the oceanic phytoplankton, which provide information about past ecological and climatological variability. Results are supported by data on fossil organic compounds (sea surface temperatures, alkenones, and n-hexacosan-1-ol index) and geochemical analyses (benthic δ13Ccc and planktonic δ18Occ isotopes). Three scenarios are taken into account for this location at centennial-scale resolution over the last 70,000 years: the Holocene and the stadial and interstadial modes. The different alternatives are described by means of elements such as nutrients; upwelling phenomena; temperatures at surface and subsurface level; or the arrival of surface turbid, fresh, and cold waters due to icebergs, low sea level, increased aridity, and dust. During the Holocene, moderate primary productivity was observed (mainly concentrated in E. huxleyi specimens); surface temperatures were at maxima while the water column was highly ventilated by northern-sourced polar deep waters and warmer subsurface, nutrient-poor subtropical waters. Over most of the last glacial stadials, surface productivity weakened (higher F. profunda and reworked specimen percentages and lower diunsaturated and triunsaturated C37 alkenones); the arrival of cold Arctic surface waters traced by tetraunsaturated C37 peaks and large E. huxleyi, together with powerful ventilated southern-sourced polar deep waters, disturbed, in all likelihood, the delicate vertical equilibrium while preventing significant upwelling mixing. Finally, during the last glacial interstadials (lower F. profunda percentages, nonreworked material, and higher diunsaturated and triunsaturated C37 alkenones) a combined signal is observed: warm surface temperatures were concurrent with generally low oxygenation of the deep-sea floor, moderate arrival of northern-sourced deep waters, and subsurface cold, nutrient-rich, recently upwelled waters, probably of polar origin; these particular conditions may have promoted vertical mixing while enhancing surface primary productivity (mainly of Gephyrocapsa specimens).

Late Quaternary palaeoceanography of the Circumpolar Deep Water from the South Tasman Rise

AbstractWe use sediment cores from the South Tasman Rise (STR) to reconstruct deep‐ water circulation in the southwest Pacific sector of the Southern Ocean. Sediment cores MD972106 (45° 09′ S, 146° 17′ E, 3310 m water depth) and GC34 (45° 06′ S, 147° 45′ E, 4002 m water depth) preserve records covering the last 160 k yr, with chronology controlled by calibrated accelerator mass spectrometry radiocarbon dates and benthic foraminiferal δ18O tied to SPECMAP. The STR benthic foraminiferal δ13C records provide new δ13C values for Southern Ocean deep water spanning the last 160 k yr at sites unlikely to be affected by variations in productivity. The records establish that glacial benthic foraminifera (Cibicidoides spp.) δ13C values are lower relative to interglacial values and are comparable to previous glacial benthic δ13C records in the Indian and Pacific sectors of the Southern Ocean. Comparisons of the benthic foraminiferal δ13C time series at the STR are made with the equatorial Pacific (V19‐30 and Site 846) and the equatorial Atlantic (GeoB1115). The similarity of benthic δ13C records at the STR to the equatorial Pacific suggest the Southern Ocean deep‐water mass closely tracked those of the deep Pacific, and the presence of a δ13C gradient between the STR and the equatorial Atlantic suggests there was continual production of northern source deep water over the past 160 k yr. Copyright © 2006 John Wiley & Sons, Ltd.

Evidence from the high‐latitude North Atlantic for variations in Antarctic Intermediate water flow during the last deglaciation

We present evidence that the characteristic chemical signature (based on coupled benthic foraminiferal Cd/Ca and δ13C) of Antarctic Intermediate waters (AAIW) penetrated throughout the intermediate depths of the Atlantic basin to the high‐latitude North Atlantic during the abrupt cooling events of the last deglaciation: Heinrich 1 and the Younger Dryas. AAIW may play the dynamic counterpart to the “bipolar seesaw” when near‐freezing salty bottom waters from the Antarctic (AABW) sluggishly ventilate the deep ocean. Our data reinforce the concept that interglacial circulation is stabilized by salinity feedbacks between salty northern sourced deep waters (NADW) and fresh southern sourced waters (AABW and AAIW). Further, the glacial ocean may be susceptible to the more finely balanced relative densities of NADW and AAIW, due to either freshwater input or a reversal of the salinity gradient, such that the ocean is poised for NADW collapse via a negative salinity feedback. The unstable climate of the glacial period and its termination may arise from the closer competition for ubiquity at intermediate depths between northern and southern sourced intermediate waters.

Towards an understanding of the Neogene tectonostratigraphic framework of the NE Atlantic margin between Ireland and the Faroe Islands

The Neogene succession preserved on the continental margin between Ireland and the Faroe Islands can be divided into two megasequences of Miocene and Pliocene–Holocene age. The base of each megasequence is marked by a regional unconformity. These are of latest Oligocene/early Miocene and early Pliocene age respectively and reflect major phases of Neogene margin evolution. The megasequences and their bounding unconformities reflect a gross two-stage depositional history; predominantly a response to intra-plate tectonism that modified sedimentation patterns and palaeoceanographic circulation. The latest Oligocene/early Miocene event marks a major change in the oceanographic regime, and can be linked to the establishment of deep-water exchange between the Arctic and North Atlantic oceans. The submergence of the Greenland–Scotland Ridge, which includes the Iceland–Faroe Rise and the Wyville–Thomson Ridge in the area of study, resulted in locally vigorous deep-water erosion as northern source deep water flowed through the Faroe conduit, which is part of the Southern gateway for deep-water exchange. Coeval deposition of sediment drifts occurred in the Faroe–Shetland Channel and Rockall Trough as the deep-water currents stabilised. The early Pliocene event marked the widespread instigation of shelf-margin progradation to the north–west of Britain and Ireland, and to the east of the Faroe Islands. This event is linked to the uplift and erosion of Britain, Ireland and the Faroe Islands, and is part of a larger North Atlantic-scale uplift. Significant deep-water erosion in the Rockall Trough suggests that the water circulation pattern was also modified by this event. The formation of the key megasequence boundaries occurred during times of significant plate rearrangement, and suggests that these events may be correlatable across the whole NW European margin.

Reconstructing past particle fluxes in the tropical Atlantic Ocean

Using a research strategy analogous to modern sediment trap studies, sediment accumulation patterns on submarine rises can be interpreted in terms of past ocean chemistry and circulation. We have followed this research strategy to reconstruct the history of surface water productivity and deep‐water chemistry and circulation in the eastern equatorial Atlantic (Sierra Leone Rise) and western equatorial Atlantic (Ceara Rise) during the last glacial maximum (∼18,000 B.P.). On shallow sections of these rises, at depths with little carbonate particle degradation, we assume that the accumulation of skeletal carbonate approximates the carbonate production rate in surface water. During the last glacial maximum, the rate of carbonate productivity was lower in the eastern Atlantic than it is today, while the western Atlantic exhibited no glacial‐interglacial difference in carbonate productivity. Based on the difference in carbonate accumulation rates between shallow and deep cores on the rises, we observed greater dissolution in both basins during the last glaciation. The eastern Atlantic always had a lower rate of dissolution than the western Atlantic, despite having deep water with a lower δ13C during the last glacial maximum. Organic carbon accumulation in the eastern Atlantic increased with depth in the water column during the last glaciation, suggesting that there was a bathymetric decrease in dissolved oxygen concentration at that time. These observations are consistent with a glacial decrease in the production rate of northern source deep water during the last glaciation. At that time, the mixing zone between northern source and southern source deep water migrated to the north in the Atlantic, resulting in a greater proportion of corrosive, southern source deep water in the western Atlantic, and entering the eastern Atlantic through low‐latitude fracture zones. Today the ratio of northern and southern components entering the eastern Atlantic is about 4:1. Based on δ13C and carbonate accumulation, our best estimate of this ratio during the last glaciation is 1:1.