Southern Drake Passage Research Articles

The effects of continental margins and water mass circulation on the distribution of dissolved aluminum and manganese in Drake Passage

A total of 232 samples were analyzed for concentrations of dissolved aluminum ([Al]) and manganese ([Mn]) in Drake Passage. Both [Al] and [Mn] were extremely low (∼0.3 and 0.1 nM, respectively) in the surface layer of the middle Drake Passage, most likely due to limited input and biological uptake/scavenging. Elevated [Al] (>14 nM) and [Mn] (>2 nM) over the South American continental shelf are related to land run‐off, whereas elevated concentrations (>1 nM and >2 nM, respectively) near the Antarctic Peninsula are most likely related to sediment re‐suspension. Re‐suspension of sedimentary particles and pore waters influences the distribution of [Al] and [Mn] over the continental slopes on both sides of Drake Passage. The influence of the continental margins and accumulated dust input potentially explains the higher [Al] observed eastward in the Atlantic section of the Southern Ocean. In the northern Drake Passage, elevated [Al] (∼0.8 nM) and [Mn] (∼0.3 nM) near the seafloor are most likely the result of bottom sediment re‐suspension by the relatively strong currents. In the deep southern Drake Passage sediment re‐suspension associated with the inflow of Weddell Sea Deep Water appears to cause elevated [Al] (>1 nM) and [Mn] (∼0.4 nM). In the deep northern Drake Passage, North Atlantic Deep Water brings in elevated [Al] and Southeast Pacific Deep Slope Water brings in the signature of Pacific hydrothermal vents. Elevated [Mn] andδ3He were correlated in this water layer and are most likely originating from the volcanically active ridges in the Pacific Ocean.

Habitat use of hourglass dolphins near the South Shetland Islands, Antarctica

The hourglass dolphin Lagenorhynchus cruciger is the only regularly occurring small delphinid found south of the Antarctic Polar Front, yet little is known about its ecology and habitat use. This study uses 8 years (14 cruises) of standardized shipboard surveys during January–March (2003–2011) in southern Drake Passage near the South Shetland Islands to summarize the spatial distribution of hourglass dolphin sightings and quantify habitat use. Sighting data are linked to bathymetry (depth, slope) and distance to the average location of oceanographic features. A generalized linear model is used to examine the relationships between sightings and habitat features. Hourglass dolphins were sighted on 50% of surveys (n = 29); sightings were concentrated in February. Group size tended to be 2–6 individuals; there were only 2 sightings of larger groups, of 15 and 25 individuals. Sightings were distributed entirely within the deep pelagic waters north of the South Shetland Islands in southern Drake Passage and were closely associated with the southern boundary of the Antarctic Circumpolar Current. Information on occurrence and distribution reported in this study may be useful for refining habitat associations for hourglass dolphins at regional scales in the Southern Ocean.

Southern Ocean natural iron fertilization

Modeling and Synthesis of Southern Ocean Natural Iron Fertilization; Woods Hole, Massachusetts, 27–29 June 2011; For many years a major paradox in ocean science was the existence of regions where the major nutrients are present in nonlimiting concentrations yet phytoplankton biomass is low. Pioneering experiments in the 1990s firmly established that the likely cause of this high‐nutrient, low‐chlorophyll condition is a deficit of iron relative to other nutrients. Iron is required for numerous processes within the cell, including photosynthesis, respiration, and nutrient uptake, yet because of its chemical properties, in seawater it is present at vanishingly small concentration levels. Elucidating the role of iron in governing ecosystem functioning and carbon sequestration is in its infancy; however, one promising approach is to make observations in regions where landmasses act as point sources of iron. In 2004–2006, three separate expeditions targeted the southern Indian Ocean around the Crozet and Kerguelen Islands and in the southern Scotia Sea around the southern Drake Passage. Representatives from all three programs met recently to compare findings and identify critical gaps in existing knowledge.

Dynamics of the current system in the southern Drake Passage

It has long been seen from satellite ocean color data that strong zonal gradients of phytoplankton biomass persistently occur in the southern Drake Passage during austral summer and fall, where the low productivity Antarctic Surface Water (ASW) within the Antarctic Circumpolar Current (ACC) region transforms to the high productivity water. An interdisciplinary cruise was conducted in February and March 2004 to investigate potential physical and biogeochemical processes, which are responsible for transporting nutrients and metals and for enhancing primary production. To explore physical processes at both the meso- and large-scales, surface drifters, a shipboard Acoustic Doppler Current Profiler and conductivity–temperature–depth sensors were used. Analyzing meso- and large-scale hydrography, circulation and eddy activities, it is shown that the topographic rise of the Shackleton Transverse Ridge plays the key role in steering an ACC branch southward west of the ridge, forming an eastward ACC jet through the gap between the ridge and Elephant Island and causing the offshelf transport of shelf waters approximately 1.2 Sv from the shelf near Elephant Island. High mesoscale eddy activities associated with this ACC southern branch and shelf waters transported off the shelf were found. The mixing between the iron-poor warmer ASW of the ACC and iron-rich waters on the shelf through horizontal transport and vertical upwelling processes provides a physical process which could be responsible for the enhanced primary productivity in this region and the southern Scotia Sea.

Shelf‐derived iron inputs drive biological productivity in the southern Drake Passage

In the Southern Ocean near the Antarctic Peninsula, Antarctic Circumpolar Current (ACC) fronts interact with shelf waters facilitating lateral transport of shelf‐derived components such as iron into high‐nutrient offshore regions. To trace these shelf‐derived components and estimate lateral mixing rates of shelf water, we used naturally occurring radium isotopes. Short‐lived radium isotopes were used to quantify the rates of shelf water entrainment while Fe/228Ra ratios were used to calculate the Fe flux. In the summer of 2006 we found rapid mixing and significant lateral iron export, namely, a dissolved iron flux of 1.1 × 105 mol d−1 and total acid leachable iron flux of 1.1 × 106 mol d−1 all of which is transported in the mixed layer from the shelf region offshore. This dissolved iron flux is significant, especially considering that the bloom observed in the offshore region (0.5–2 mg chl a m−3) had an iron demand of 1.1 to 4 × 105 mol Fe. Net vertical export fluxes of particulate Fe derived from 234Th/238U disequilibrium and Fe/234Th ratios accounted for only about 25% of the dissolved iron flux. On the other hand, vertical upward mixing of iron rich deeper waters provided only 7% of the lateral dissolved iron flux. We found that similarly to other studies in iron‐fertilized regions of the Southern Ocean, lateral fluxes overwhelm vertical inputs and vertical export from the water column and support significant phytoplankton blooms in the offshore regions of the Drake Passage.

Major Role of Microbes in Carbon Fluxes during Austral Winter in the Southern Drake Passage

Carbon cycling in Southern Ocean is a major issue in climate change, hence the need to understand the role of biota in the regulation of carbon fixation and cycling. Southern Ocean is a heterogeneous system, characterized by a strong seasonality, due to long dark winter. Yet, currently little is known about biogeochemical dynamics during this season, particularly in the deeper part of the ocean. We studied bacterial communities and processes in summer and winter cruises in the southern Drake Passage. Here we show that in winter, when the primary production is greatly reduced, Bacteria and Archaea become the major producers of biogenic particles, at the expense of dissolved organic carbon drawdown. Heterotrophic production and chemoautotrophic CO2 fixation rates were substantial, also in deep water, and bacterial populations were controlled by protists and viruses. A dynamic food web is also consistent with the observed temporal and spatial variations in archaeal and bacterial communities that might exploit various niches. Thus, Southern Ocean microbial loop may substantially maintain a wintertime food web and system respiration at the expense of summer produced DOC as well as regenerate nutrients and iron. Our findings have important implications for Southern Ocean ecosystem functioning and carbon cycle and its manipulation by iron enrichment to achieve net sequestration of atmospheric CO2.

Core-based reconstruction of paleoenvironmental conditions in the southern Drake Passage (West Antarctica) over the last 150 ka

A deep-sea sediment core (GC98-06) from the southernmost Drake Passage, West Antarctica, shows late Quaternary depositional environments distinctly different from sedimentary drifts commonly found along the southwestern Pacific margin of the Drake Passage. The chronology of the core has been inferred using geochemical tracers of paleoproductivity and diatom biostratigraphy, and represents the paleoceanographic conditions in a continental rise setting during the last 150,000 years. Three dominant sediment types associated with distinct sedimentary processes have been identified using textural/compositional analyses: (1) hemipelagic mud (interglacial sediments) deposited from pelagic settling of bioclasts, meltwater plumes, and ice-rafted detritus; (2) terrigenous mud (glacial sediments) delivered by turbid meltwater plumes; and (3) massive muds marking the boundaries from interglacial to glacial periods. The succession of the sedimentary facies in core GC98-06 is interpreted to reflect temporal changes in environmental conditions prevailing on the continental rise of the southern Drake Passage in the course of successive climatic stages over the last 150 ka: from the bottom upward, these are glacial, interglacial, glaciation, glacial, and interglacial episodes. Variability in sediment flux and diatom abundance seem to have been related to changes in glacial advance, sea-ice extent, and specific sedimentary environments, collectively influenced by mid- to late Quaternary climatic changes.

Weddell Sea anomalies: Excitation, propagation, and possible consequences

Antarctic marginal seas are susceptible to significant decadal variability as revealed by the analysis of a 200‐year integration of a regional ice‐ocean model forced with the atmospheric output of the IPCC climate model ECHAM5‐MPIOM. The strongest signal occurs on the southern and western Weddell Sea continental shelf where changes in bottom salinity are initiated by a variable sea ice cover and modification of surface waters near the Greenwich meridian. Related zonal shifts of the western rim current guide deep waters with different temperature out of the Weddell Sea. With a deep boundary current the temperature signal propagates westward through southern Drake Passage and along the upper continental rise in the southeast Pacific thereby influencing the hydrographic conditions on the continental shelf of Bellingshausen, Amundsen, and Ross Seas.

Long-term trends and interannual variability of temperature in Drake Passage

Temperature data collected over the last 36 years (1969–2004) in Drake Passage are used to examine interannual temperature variation and long-term trends in the upper ocean. To reduce the effect of variation from different sampling locations and temporal variability introduced by meridional shifts in the Polar Front (PF), the data were divided into two sub-regions north (∼3800 temperature profiles) and south (∼3400) of the PF. Temperature anomalies were formed by removing a temporal mean field for each profile in each sub-region at 100 m depth intervals from the surface to 700 m. North of the PF, statistically significant warming trends of ∼0.02 °C yr −1 were observed that were largely depth-independent between 100 and 700 m. A statistically significant cooling trend of −0.07 °C yr −1 was observed at the surface south of the PF, which was smaller (−0.04 °C yr −1) but still statistically significant when possible seasonal sampling biases were accounted for. The observed cooling at the surface and warming at depth is largely consistent with a poleward shift of the PF due to enhancement of westerly winds in the Southern Ocean, as recently suggested by models and observations. The observed annual temperature anomalies in the upper 400 m north of the PF and in the upper 100 m south of the PF are highly correlated to variability in sea ice, and also to climate indices of the Antarctic Oscillation and the El Niño Southern Oscillation. Variability in sea ice and temperature anomalies lag El Niño variability in the Pacific, with a phasing consistent with the observed cyclical patterns of sea ice and sea surface temperature associated with the Antarctic Circumpolar Wave or Antarctic Dipole Mode in the Southern Ocean. In contrast, the sea ice variability and temperature anomalies at all depths north of the PF and at 0–100 m depth south of the PF were primarily coincident with, or led the Antarctic Oscillation Index. No significant correlations were found with the large-scale climate variability indices in southern Drake Passage below 100 m depth, which is occupied by upper Circumpolar Deep Water (uCDW). This water mass is not formed locally, is largely isolated from the surface, and exhibits vertical and lateral homogeneity. Hence changes may be difficult to detect in the available measurements, and climate variation in the source water regions of uCDW may take a long time to reach Drake Passage.

Circulation in the Ona Basin, southern Drake Passage

Five Argo floats trapped in the Ona Basin, in the southern Drake Passage, provided unprecedented information on the water masses and the circulation present in all the seasons from March 2002 to April 2006. Their profiles recorded evidence of deep‐penetrating eddies, interleaving of water masses, and branching of the Southern Front of the Antarctic Circumpolar Current as it meets the Shackleton Fracture Zone. Ten‐day mean‐velocity estimates at the float parking depth (2000 db) often exceeded 15 cm s−1, revealing the intensity of the mesoscale field. Satellite data showed that the trapping period of the floats corresponded to the presence of an anticyclonic meander of the polar front over a depression in the topography to the northwest of the Ona Basin. The presence of this meander seems to be linked to low‐frequency modes of variability in sea surface height in the Yaghan Basin.

Iron limitation across chlorophyll gradients in the southern Drake Passage: Phytoplankton responses to iron addition and photosynthetic indicators of iron stress

Processes influencing phytoplankton bloom development in the southern Drake Passage were studied using shipboard iron‐enrichment incubations conducted across a surface chlorophyll gradient near the Antarctic Peninsula, in a region of water mass mixing. Iron incubation assays showed that Antarctic Circumpolar Current (ACC) waters were severely iron limited, while shelf waters with high ambient iron concentrations (1–2 nmol L−1) were iron replete, demonstrating that mixing of the two water masses is a plausible mechanism for generation of the high phytoplankton biomass observed downstream of the Antarctic Peninsula. In downstream highchlorophyll mixed waters, phytoplankton growth rates were also iron limited, although responses to iron addition were generally more moderate as compared to ACC waters. Synthesizing results from all experiments, significant correlations were found between the initial measurements of Photosystem II (PSII) parameters (Fv:Fm, σPSII, and p) and the subsequent responses of these waters to iron addition. These correlations indicate that PSII parameters can be used to assess the degree of iron stress experienced in these waters and likely in other regions where photoinhibition and nitrogen stress are not confounding factors.

On the detectability of internal tides in Drake Passage

The Southern Ocean hosts significant topographic mixing that might be associated with internal tides. Tidal signals are evident in bottom temperature at 1000 m in Drake Passage, suggesting that internal tides with an amplitude of between ∼20 and 200 m may be present. Various necessary conditions for internal tide generation show that the steep topography in and around Drake Passage can initiate internal tides, and recent global tide models have suggested this region to generate very large interface displacements. Here, we present an attempt to detect internal tides in Drake Passage. During the last 10 years, combinations of bottom pressure recorders and inverted echo sounders have been deployed in the region. The bottom pressure recorders measure predominantly the barotropic tide; the inverted echo sounders measure travel time from sea bed to sea surface and therefore are influenced both by sea level (barotropic tide) and internal sound speed (internal tide). By subtracting one from the other, the internal tide should be detectable. Although the technique works successfully around Hawaii, it does not prove the existence of large internal tides in Drake Passage. The detectability of the internal tidal signal in Drake Passage is investigated using a six-layer one-dimensional model to simulate the bottom pressure and travel time signals of a semi-diurnal tide. The temperature and salinity stratification in Drake Passage is sufficiently weak that large vertical excursions are necessary to produce a signal in travel time detectable above the noise in Drake Passage. An internal tide of at least 70 or 20 m in northern and southern Drake Passage, respectively, would be detected. The fact that these are, perhaps surprisingly, not detected by the combination of bottom pressure and travel time, constrains the internal tides in Drake Passage to be ∼20 m in southern Drake Passage, and between 20 and 70 m in northern Drake Passage. The model also predicts that satellite altimetry would not be able to detect internal tides in Drake Passage, but would in the Brazil Basin and Hawaii regions.

Rich and rare—First insights into species diversity and abundance of Antarctic abyssal Gastropoda (Mollusca)

Abstract The abyssal depths of the polar oceans are thought to be low in diversity compared with the shallower polar shelves and temperate and tropical deep-sea basins. Our recent study on the gastropod fauna of the deep Southern Ocean gives evidence of the existence of a rich gastropod assemblage at abyssal depths. During the ANDEEP I and II expeditions to the southern Drake Passage, Northwestern Weddell Sea, and South Sandwich Trench, gastropods were collected by bottom and Agassiz trawls, epibenthic sledge, and multicorer, at 40 stations in depths between 127 and 5194 m. On the whole, 473 specimens, corresponding to 93 species of 36 families, were obtained. Of those, 414 specimens were caught below 750 m depth and refer to 84 (90%) benthic species of 32 (89%) families. Most families were represented by a single species only. The numerically dominant families were Skeneidae and Buccinidae (with 10 and 11 species, respectively), Eulimidae and Trochidae (with 9 species each), and Turridae (6 species). Thirty-Seven benthic deep-sea species (44%) were represented by a single specimen, and another 20 species (24%) were found at a single station, suggesting that more than two thirds of Antarctic deep-sea gastropod species are very rare or have a very scattered distribution. Of the 27 species occurring at two or more deep-sea stations, 14 were collected with different gear. Approximately half of the deep-water species are new to science or have been recently described. The present investigation increases the total number of recorded benthic Antarctic deep-sea gastropods (below 750 m) from 115 to 177. The previously known depth ranges have been extended, often considerably, for 31 species. The collected deep-sea gastropods comprise both eurybathic shelf species (29%) and apparently true deep-sea species (58%); some of the latter may belong to a so far unknown Antarctic abyssal fauna. Geographical ranges of the collected Antarctic benthic deep-sea gastropod species appear limited, and all these 84 species seem endemic to Antarctica south of the Polar Front. Comparing diversity and abundances based on epibenthic sledge samples, there is no clear relationship between Antarctic deep-sea gastropod abundance and species richness with depth. However, both Antarctic and adjacent deep-sea areas are still far from being adequately sampled to allow more comprehensive conclusions.

Quasi-geostrophic 3D circulation and mass transport in the western Bransfield Strait during Austral summer 1995/96

In the framework of the FRUELA carbon-flux study, two oceanographic surveys (December 1995 and January 1996) were conducted by B.I.O. Hesperides in the western basin of the Bransfield Strait and southern Drake Passage. The horizontal (geostrophic) velocity field is characterized by two jet-like features linked to frontal hydrographic structures: a meandering current off the continental shelf edge of the southern Drake Passage, corresponding to the Southern front of the Antarctic Circumpolar Current, and the Bransfield Current, related to the Bransfield front. The first jet shows some changes in the meandering pattern from December 1995 to January 1996, whereas the second has a very stationary location and structure. The estimated horizontal circulation and mass transports are compared with available current meter data and with previous geostrophic estimates in the context of a sensitivity analysis that takes into account the choice of the reference level and the station distribution. The vertical velocity field has been computed by integrating the quasi-geostrophic Omega equation. Maximum estimated values are of the order of 1 m/day, which suggests that a phytoplankton bloom observed in the northern part of the study area in December (and vanished one month later) should not be attributed to frontal dynamic instabilities inducing upward motion and nutricline-related lifting, but rather to other hydrographic factors

Water masses and 200 m relative geostrophic circulation in the western Bransfield Strait region

Four hydrographic surveys to 200 db of 16–20 nm horizontal resolution carried out in the vicinity of western Bransfield Strait in the period November 1986 to March 1987 are used to provide a description of water masses and relative geostrophic circulation in the survey area. The water mass structure is characterized by two frontal structures with a complex vertical and horizontal interleaving of water from the Southern Drake Passage, Orkney Island shelf, Bellingshausen Sea and the shelf of Antarctic Peninsula. In the northwestern segment of the survey area, intrusions of Drake Passage water into the Bransfield are found beneath a frontal zone at a 100-m depth between Snow and Smith Islands. In the southeastern segment fresh, warm water from the Gerlache Straits flows over the cold salty water from the Antarctic Peninsula. A geostrophically balanced relative flow, termed the Bransfield Current, transports water along a second front between Bellingshausen Sea and Antarctic Peninsula from the Bellingshausen Sea and Gerlache Straits to the northeast. A cyclonic gyre exists around Low Island. In this current, the 200 db relative geostrophic mean current maximum is 8 cm s −1 and the geostrophic eddy kinetic energy is 18–55 cm 2 s −2, being strongest near the flow axis. Based on estimates of other mid-latitude ocean current areas with similar mean flows and eddy energies, the single particle diffusion coefficient is 1–3 × 10 7 cm 2 s −1. This circulation and diffusion could transport particles from the biologically rich Gerlache Strait to Livingston Island and the Drake Passage in 15–30 days, while particles can spread an average ± 50 km to either side of a track traced out by the mean flow.

The distribution, abundance and community structure of marine birds in southern Drake Passage and Bransfield Strait, Antarctica

We studied the distribution and abundance of marine birds in Bransfield Strait and southern Drake Passage during January and February 1985. We identified clusters of bird species, “communities” with similar distributions, and examined the extent to which the distributions of these seabird communities reflected the underlying physical oceanography of the region. Based on temperature, salinity and silicon data, we identified 11 water masses which resulted from varying degrees of mixing of water from the Pacific Basin, the Bellingshausen Sea, the Weddell Sea and terrestrial runoff. Seabird species assemblages were associated with contiguous clusters of water masses, suggesting a response by the birds to apparently subtle differences in their marine habitat.

A study of the sulfur cycle in the Antarctic marine boundary layer

Atmospheric sampling was conducted aboard R/V Polar Duke from March 21 to April 27, 1986, in the southern Drake Passage and in the coastal waters west of the Antarctic Peninsula. Average atmospheric concentrations (n = 6) of sulfur dioxide (SO2) vapor, aerosol methanesulfonate (CH3SO3−), and aerosol non‐sea‐salt sulfate (nss−SO4=) were 0.36, 0.22, and 0.34 nmol m−3, respectively. Cascade impactor data indicated that more than 85% of the masses of both CH3SO3− and nss−SO4= were on particles of <0.25‐μm mean aerodynamic radius. Average concentrations (n = 8) of CH3SO3− and nss−SO4= in precipitation were 0.09 and 0.57 μM, respectively. Box model calculations based on this limited data suggest a total sulfur removal rate from the local marine boundary layer of 0.3–7.2 μmol m−2 day−1. This removal approximately balances the oceanic source of dimethylsulfide (DMS) sulfur of 1.2–4.4 μmol m−2 day−1 (±100%) estimated by Berresheim [1987] from data obtained during the same cruise. The box model calculations also suggest that heterogeneous SO2 oxidation may be the major pathway for DMS to sulfate conversion (∼75%) and/or that other sources of sulfate may not be negligible. Observed mineral aerosol and 222Rn concentrations were low but slightly elevated above “pure” remote marine background levels, suggesting a weak but discernible continental character in sampled air. Uncertainties are large owing to the limited sampling time, study area, and numbers of samples obtained, but overall, the sulfur cycle in the coastal Antarctic marine boundary layer appears not to differ remarkably from that in other remote marine regions.

Relationships between the distributions of marine avian predators and their prey, Euphausia superba, in Bransfield Strait and southern Drake Passage, Antarctica

Relationships between the distributions of marine avian predators and their prey, Euphausia superba, in Bransfield Strait and southern Drake Passage, Antarctica

3He in the Bransfield Strait waters: indication for local injection from back-arc rifting

Helium data from the waters of the Bransfield Strait, the southern Drake Passage and the northwestern shelf of the Weddell Sea are presented. The 3He profiles from the eastern and central basins of the Bransfield Strait show maxima ( δ 3He ≈ 7%) below the sill depths that separate the strait from the surrounding open ocean. The 3He excess is interpreted as a local injection of a 3He-rich helium component into the deep waters of the Bransfield Strait from backarc rifting. Tritiogenic 3He and excess 3He from mixing with Circumpolar Deep Water are excluded as possible sources. The estimated 3He/ 4He ratio of the injected helium component (2.4–5.0 × 10 −6) is less than that of pure mantle helium and may contain radiogenic helium from continental crustal material which underlies the Bransfield Strait.

Comparisons of sea level and bottom pressure measurements at Drake Passage

Tide gauge and atmospheric sea level pressure measurements from each side of Drake Passage are compared with multiyear time series of bottom pressure at 500 m depth. Synthetic subsurface pressure (SSP), smoothed with a 40‐hour low‐pass filter, derived from tidal and sea level pressure records from the British Antarctic Survey base Faraday is coherent with bottom pressure from southern Drake Passage for periods of 6 to 600 days, but it is about 60° out‐of‐phase at the annual period. The difference between the two series for periods longer than 60 days is mainly annual and is as large as, or larger than, the seasonal variability of bottom pressure itself. Annual variations in open ocean mixed‐layer density near Antarctica are too small to account for the difference. The Faraday SSP series spans 21 years, slightly longer than the 18.6‐year lunar nodal tide, which appears to exist in the record. At the northern side of Drake Passage, SSP from Puerto Williams, Chile, does not compare well with nearby bottom pressure for any band of periods, apparently due to local winds for periods under 100 days and to annual cycles in upper layer density. Barotropic changes in transport of the Antarctic Circumpolar Current cannot be directly estimated using these surface observations, even though over seasonal periods they are an order of magnitude larger than the baroclinic changes due to variations in upper layer density.