The Particulate Organic Carbon-to-Nitrogen Ratio Varies With Ocean Currents

Ocean currents could adjust ocean carbon and nitrogen composition which are an important part of the global carbon and nitrogen cycle. We procured global concentrations of particulate carbon and nitrogen in different depths, classified them according to ocean currents (upper 300 m), and analyzed POC-to-PON ratio (particulate organic carbon-to-nitrogen ratio) variations. We found that the regions with currents have a higher ratio than those without currents in the northern hemisphere, except in 50°–60°N (median ratio without currents is 8.38). Warm currents (median ratio ranges from 5.96 to 8.44) have a higher ratio than cold currents (6.19–8.89), except for the East Greenland Current (reach to 8.44) and Labrador Current (reach to 8.89). Meanwhile, we also analyzed the effects of ocean currents’ flowing and found that the distributions of the POC-to-PON ratio vary in different current types (e.g., cause of formation and distance from the shore). Generally speaking, the POC-to-PON ratio of the eolian currents and near-ocean currents change fiercer than that of compensation currents and near-coast currents. Ocean currents also have a buffering effect in the variation between surface and deep water, which prevents the severe change of the POC-to-PON ratio. The high-value anomaly of POC-to-PON caused by the confluence of warm and cold currents was also analyzed. It can be deduced that the high ratio in the high-latitude region was mainly caused by the terrigenous organic matter (especially carbon) and low nitrogen.

Holocene paleoceanography of the Northeast Greenland shelf

The Northeast Greenland shelf is highly sensitive to climate and ocean variability because it is swept by the East Greenland Current, which, through the western Fram Strait, forms the main pathway of export of sea ice and cold water masses from the Arctic Ocean into the North Atlantic Ocean. We carried out benthic foraminiferal assemblage, stable isotope- and sedimentological analyses of a marine sediment core retrieved from the Northeast Greenland shelf (core DA17-NG-ST7-73), which provided a multiproxy reconstruction of Holocene paleoceanographic conditions. The results reveal significant variations in the water masses and thus, in the strength of the East Greenland Current over the last ca. 9.4 ka BP. Between 9.4 and 8.2 ka BP the water column off Northeast Greenland was highly stratified, with cold, sea ice-loaded surface waters and strong influx of warm Atlantic Water in the subsurface. At ~8.4 ka BP a short-lived peak in terrestrial elements may be linked to influx of ice-berg transported sediments and thus, to the so-called 8.2 ka event. Holocene Thermal Maximum like conditions prevailed from 8.2 to 6.2 ka BP, with a strong influence of the Return Atlantic Current and a weakened transport of Polar Water in the upper East Greenland Current. After 6.2 ka BP we recorded a return to a more stratified water column with sea-ice loaded surface waters and still Atlantic-sourced subsurface waters. After 4.2 ka BP increased Polar Water at the surface of the East Greenland Current and reduction of the Return Atlantic Water at subsurface levels led to freshening and reduced stratification of the water column and (near) perennial sea-ice cover. The Neoglaciation started at 3.2 ka BP at our location, characterized by a strengthened East Greenland Current. Cold subsurface water conditions with possible sea-ice cover and minimum surface water productivity persisted here throughout the last ~3 kyr.

Freshwater export from the south-east Greenland shelf into the Irminger Sea and relation to wind events

<p>Greenland Ice Sheet melt and freshening of the Arctic Ocean lead to increased discharge of freshwater into the East Greenland Current. If this additional freshwater reaches the convective regions of the Subpolar North Atlantic it could weaken deep mixing and affect the strength of the Atlantic Meridional Overturning Circulation. In particular, freshwater exported away from the South-East Greenland shelf could affect deep convection in the Irminger Sea, which has recently been shown to have a key role in the Atlantic overturning circulation. Though export of fresh shelf surface water is well observed west of Greenland, there is still little insight into surface water export from the East Greenland shelf to the Irminger Sea.</p><p>The East Greenland Current Drifter Investigation of Freshwater Transport drifter deployment conducted in August 2019 at 65°N on the eastern side of Greenland, resulted in five out of 30 drifters being exported away from the east Greenland shelf, four of which were exported at Cape Farewell. The specific wind regime at Cape Farewell is a potential driving factor for enhanced freshwater export in the area. While persistent south-eastward barrier winds push surface waters to the coast over most of the eastern shelf, Cape Farewell experiences strong eastward wind events such as tip-jets that could cause off-shelf export. Using wind data from the ERA-5 atmospheric reanalysis, we compute Ekman transport along the east Greenland shelf. We find greater probability for off-shelf Ekman transport at Cape Farewell than along the rest of the shelf, confirming that the area is the most likely to contribute to wind-driven freshwater export to the Irminger Sea. Wind and surface velocity data from a high-resolution model (2 km) are used to further investigate and quantify freshwater export at Cape Farewell and how it relates to local wind events.</p>

Annual variability of the long-lived anthropogenic radionuclides 129I and 236U in the Fram Strait and their use as water mass composition tracers

<p>Anthropogenic chemical tracers are powerful tools to study pathways, water mass provenance and mixing processes in the ocean. Releases of the long-lived anthropogenic radionuclides <sup>129</sup>I and <sup>236</sup>U from European nuclear reprocessing plants label Atlantic Water entering the Arctic Ocean with a distinct signal that can be used to track pathways and timescales of Atlantic Water circulation in the Arctic Ocean and Fram Strait. Apart from their application as transient tracers, the difference in anthropogenic radionuclide concentrations between Atlantic- and Pacific-origin water provides an instrument to distinguish the interface between both water masses. In contrast to classically used water mass tracers such as nitrate-phosphate (N:P) ratios, the two radionuclides are considered to behave conservatively in seawater and are not affected by biogeochemical processes occurring in particular in the broad shelf regions of the Arctic Ocean.</p><p>Here we present a time-series of <sup>129</sup>I and <sup>236</sup>U data across the Fram Strait, collected in 2016 (as part of the GEOTRACES program) and in 2018 and 2019 (by the Norwegian Polar Institute). While the overall spatial distribution of both radionuclides was similar among the three sampling years, significant differences were observed in the upper water column of the EGC, especially between 2016 and 2018. This study is the first attempt to investigate the potential of <sup>129</sup>I and <sup>236</sup>U as water mass composition tracers in the East Greenland Current (EGC). We discuss how the <sup>129</sup>I - <sup>236</sup>U tracer pair can be applied to estimate fractions of Atlantic and Pacific Water, especially considering their time-dependent input into the Arctic Ocean.</p>

Submarine Melt Water from the 79 North Glacier (79NG, Nioghalvfjerdsbræ), northeast Greenland

<p>The Greenland Ice Sheet faces accelerated melting under warming climate conditions. The involved processes are surface melting, iceberg calving, and submarine melting through the contact of warm water with marine terminating glaciers. The Nioghalvfjerdsfjorden Glacier (79 North Glacier, 79NG) is the largest marine terminating outlet glacier of the Northeast Greenland Ice Stream and has still a floating ice tongue. In the cavity, the heat of inflowing warm and saline Atlantic Water melts the floating ice shelf at its base, and the colder and fresher outflow is exported towards the shelf break and presumably south with the East Greenland Current. However, freshwater from submarine melting is hardly distinguishable from other freshwater sources off the sources by salinity alone. To identify and to quantify the fraction and distribution of submarine melt water on the northeast Greenland shelf, we use helium (He) and neon (Ne) observations, obtained directly at the calving front of the 79NG, in its close and far vicinity on the northeast Greenland shelf, and beyond the shelf break in Fram Strait during a Polarstern expedition in 2016. These lighter and low soluble noble gasses provide a unique tool to identify submarine melt water and to quantify its fractions. We calculate a submarine melt water formation rate of 14.5 ± 2.3 Gt per year, equivalent to a basal melt rate of 8.6 ± 1.4 m per year of the 79NG. Submarine melt water fractions are present on the shelf, but dilute from 1.8% at the 79NG calving front to nonsignificant in Fram Strait. A surplus of Ne on most of the shelf region indicates that up to 10% of the original water mass had been transformed to sea ice.</p>

Seasonal and Mesoscale Variability of the Two Atlantic Water Recirculation Pathways in Fram Strait

<p>Atlantic Water, which is transported northward by the West Spitsbergen Current, partly recirculates (i.e. turns westward) in Fram Strait. This determines how much heat and salt reaches the Arctic Ocean, and how much joins the East Greenland Current on its southward path. We describe the Atlantic Water recirculation's location, seasonality, and mesoscale variability by analyzing the first observations from moored instruments at five latitudes in central Fram Strait, spanning a period from August 2016 to July 2018. We observe recirculation on the prime meridian at 78°50'N and 80°10'N, respectively south and north of the Molly Hole, and no recirculation further south at 78°10'N and further north at 80°50'N. At a fifth mooring location at 79°30'N, we observe some influence of the two recirculation branches. The southern recirculation is observed as a continuous westward flow that carries Atlantic Water throughout the year, though it may be subject to broadening and narrowing. It is affected by eddies in spring, likely due to the seasonality of mesoscale instability in the West Spitsbergen Current. The northern recirculation is observed solely as passing eddies on the prime meridian, which are strongest during late autumn and winter, and absent during summer. This seasonality is likely affected both by the conditions set by the West Spitsbergen Current and by the sea ice. Open ocean eddies originating from the West Spitsbergen Current interact with the sea ice edge when they subduct below the fresher, colder water. Additionally the stratification set up by sea ice presence may inhibit recirculation.</p>

Response of biological productivity to North Atlantic marine front migration during the Holocene

Marine fronts delineate the boundary between distinct water masses and, through the advection of nutrients, are important facilitators of regional productivity and biodiversity. As the modern climate continues to change, the migration of frontal zones is evident, but a lack of information about their status prior to instrumental records hinders future projections. Here, we combine data from lipid biomarkers (archaeal isoprenoid glycerol dibiphytanyl glycerol tetraethers and algal highly branched isoprenoids) with planktic and benthic foraminifera assemblages to detail the biological response of the marine Arctic and polar front migrations on the North Iceland Shelf (NIS) over the last 8 kyr. This multi-proxy approach enables us to quantify the thermal structure relating to Arctic and polar front migration and test how this influences the corresponding changes in local pelagic productivity. Our data show that following an interval of Atlantic water influence, the Arctic front and its associated high pelagic productivity migrated southeastward to the NIS by ∼6.1 ka. Following a subsequent trend in regional cooling, Polar Water from the East Greenland Current and the associated polar front spread onto the NIS by ∼3.8 ka, greatly diminishing local algal productivity through the Little Ice Age. Within the last century, the Arctic and polar fronts have moved northward back to their current positions relative to the NIS and helped stimulate the productivity that partially supports Iceland's economy. Our Holocene records from the NIS provide analogues for how the current frontal configuration and the productivity that it supports may change as global temperatures continue to rise.

Circulation timescales of Atlantic Water in the Arctic Ocean determined from anthropogenic radionuclides

The inflow of Atlantic Water to the Arctic Ocean is a crucial determinant for the future trajectory of this ocean basin with regard to warming, loss of sea ice, and ocean acidification. Yet many details of the fate and circulation of these waters within the Arctic remain unclear. Here, we use the two long-lived anthropogenic radionuclides 129I and 236U together with two age models to constrain the pathways and circulation times of Atlantic Water in the surface (10–35 m depth) and in the mid-depth Atlantic layer (250–800 m depth). We thereby benefit from the unique time-dependent tagging of Atlantic Water by these two isotopes. In the surface layer, a binary mixing model yields tracer ages of Atlantic Water between 9–16 years in the Amundsen Basin, 12–17 years in the Fram Strait (East Greenland Current), and up to 20 years in the Canada Basin, reflecting the pathways of Atlantic Water through the Arctic and their exiting through the Fram Strait. In the mid-depth Atlantic layer (250–800 m), the transit time distribution (TTD) model yields mean ages in the central Arctic ranging between 15 and 55 years, while the mode ages representing the most probable ages of the TTD range between 3 and 30 years. The estimated mean ages are overall in good agreement with previous studies using artificial radionuclides or ventilation tracers. Although we find the overall flow to be dominated by advection, the shift in the mode age towards a younger age compared to the mean age also reflects the presence of a substantial amount of lateral mixing. For applications interested in how fast signals are transported into the Arctic's interior, the mode age appears to be a suitable measure. The short mode ages obtained in this study suggest that changes in the properties of Atlantic Water will quickly spread through the Arctic Ocean and can lead to relatively rapid changes throughout the upper water column in future years.

Kinematic Structure and Dynamics of the Denmark Strait Overflow from Ship-Based Observations

The dense outflow through Denmark Strait is the largest contributor to the lower limb of the Atlantic meridional overturning circulation, yet a description of the full velocity field across the strait remains incomplete. Here we analyze a set of 22 shipboard hydrographic–velocity sections occupied along the Látrabjarg transect at the Denmark Strait sill, obtained over the time period 1993–2018. The sections provide the first complete view of the kinematic components at the sill: the shelfbreak East Greenland Current (EGC), the combined flow of the separated EGC, and the North Icelandic Jet (NIJ), and the northward-flowing North Icelandic Irminger Current (NIIC). The total mean transport of overflow water is 3.54 ± 0.29 Sv (1 Sv ≡ 106 m3 s−1), comparable to previous estimates. The dense overflow is partitioned in terms of water mass constituents and flow components. The mean transports of the two types of overflow water—Atlantic-origin Overflow Water and Arctic-origin Overflow Water—are comparable in Denmark Strait, while the merged NIJ–separated EGC transports 55% more water than the shelfbreak EGC. A significant degree of water mass exchange takes place between the branches as they converge in Denmark Strait. There are two dominant time-varying configurations of the flow that are characterized as a cyclonic state and a noncyclonic state. These appear to be wind-driven. A potential vorticity analysis indicates that the flow through Denmark Strait is subject to symmetric instability. This occurs at the top of the overflow layer, implying that the mixing/entrainment process that modifies the overflow water begins at the sill.