Southern Ocean Origin Research Articles

Novel phenolic compound from Southern Ocean microalgae Chlorella sp. PR-1 and its antibacterial activity

Marine microalgae has been attracting the researcher’s attention for centuries. Development on microalgal research is majorly favoured by its medicinal, pharmaceutical or cosmeceutical properties. The advancement in the investigation related to microalgal products have been concentrated in the coastal zones because of the greater supply of raw material. The Southern Ocean, highly productive and relatively poorly studied ecosystem, constitutes approximately 10% of the global volume of the oceans. In this study, marine microalgae Chlorella sp. PR-1 was isolated from the Southern Ocean (Indian Sector) for the identification of bioactive antibacterial compounds. The antimicrobial activity of this microalgal extracts was evaluated against three gram positive (Staphylococcus aureus, Bacillus licheniformis, and Bacillus subtilis) and three gram negative bacteria (Pseudomonas aeruginosa, Salmonella typhimurium, and Escherichia coli). The extract showing antibacterial activity was further purified by thin layer and column chromatography. The antimicrobial activity was again evaluated and confirmed with the purified fraction against the same bacteria. The identification of the functional groups in the purified fraction was performed by infrared spectroscopic analysis. Based on gas chromatographic and mass spectroscopic analysis, the principle bioactive compound was proved to be 2,4-bis (1,1-dimethylethyl)- phenol. Thus, the bioactive compound isolated from marine microalga of Southern Ocean origin may be a novel alternative source of antibacterials in the future.

Antarctic icebergs reorganize ocean circulation during Pleistocene glacials.

The dominant feature of large-scale mass transfer in the modern ocean is the Atlantic meridional overturning circulation (AMOC). The geometry and vigour of this circulation influences global climate on various timescales. Palaeoceanographic evidence suggests that during glacial periods of the past 1.5 million years the AMOC had markedly different features from today1; in the Atlantic basin, deep waters of Southern Ocean origin increased in volume while above them the core of the North Atlantic Deep Water (NADW) shoaled2. An absence of evidence on the origin of this phenomenon means that the sequence of events leading to global glacial conditions remains unclear. Here we present multi-proxy evidence showing that northward shifts in Antarctic iceberg melt in the Indian-Atlantic Southern Ocean (0-50°E) systematically preceded deep-water mass reorganizations by one to twothousand years during Pleistocene-era glaciations. With the aid of iceberg-trajectory model experiments, we demonstrate that such a shift in iceberg trajectories during glacial periods can result in a considerable redistribution of freshwater in the Southern Ocean. We suggest that this, in concert with increased sea-ice cover, enabled positive buoyancy anomalies to 'escape' into the upper limb of the AMOC, providing a teleconnection between surface Southern Ocean conditions and the formation of NADW. The magnitude and pacing of this mechanism evolved substantially across the mid-Pleistocene transition, and the coeval increase in magnitude of the 'southern escape' and deep circulation perturbations implicate this mechanism as a key feedback in the transition to the '100-kyr world', in which glacial-interglacial cycles occur at roughly 100,000-year periods.

Evidence of Southern Ocean influence into the far Northwest Pacific (Northern Emperor Rise) since the Bølling–Allerød warming

The role of the Southern Ocean in releasing CO 2 (sequestered in the global ocean during the Last Glacial Maximum) into the atmosphere during deglaciation is an important topic for investigation of Earth's climate. Changes in global deep water circulation associated with upwelling of Circumpolar Deep Water (CDW) around Antarctica may have played a part in the CO 2 release, but remain poorly studied. The potential response of the Pacific Ocean, including the North Pacific, to upwelling of CDW with its vast reservoir of CO 2 remains unresolved. Here we combine productivity proxies, oxygen and carbon isotope values in benthic and planktic foraminifera, data on the occurrence and abundance of ice-rafted debris, and benthic foraminiferal species composition in three sediment cores with published data for three cores from the Northern Emperor Rise for the Last Glacial Maximum, deglaciation and Holocene in order to elucidate the North Pacific role in CO 2 redistribution in the past. Age models of the cores are based on radiocarbon data calibrated by 14 C atmospheric plateau tuning. The calcium carbonate content in all cores increased abruptly around 14.5 ka, indicating an influx of relatively young water enriched in carbonate ion, oxygen and nutrients, and sourced in the Southern Oceans. A decrease in the extent of sea ice at the NER area during early deglaciation is reflected in sharp increases in the productivity of siliceous phytoplankton near the onset of Bølling/Allerød warming, possibly facilitated by the influx of Southern Ocean-sourced nutrient-rich waters. • Stable isotope, lithological and micropaleontological proxies of 6 cores from the Western Subarctic Gyre (WSG) were studied. • Age models of cores over the last 25kyr were modified by radiocarbon data calibrated by 14C atmospheric plateau tuning. • Multiple lines of evidences support abrupt flushing of Southern Ocean origin water into the deep WSG since 14.5 ± 0.2 ka.

Upwelling characteristics and nutrient enrichment of the Kangaroo Island upwelling region, South Australia

An analysis is presented of hydrographic and nutrient data collected over three years for the Kangaroo Island upwelling region, Lincoln Shelf, South Australia, to determine the signature of upwelled water, depth of upwelling and the source water mass being brought onto the shelf. Strong upwelling seasons were recorded during the 2007–2008 and 2009–2010 summers, while the summer of 2008–2009 had only one weak upwelling event. Strong upwelling events during February and March 2008 and February and March 2010 recorded temperatures and salinities as low as 10.4°C and 34.85, and NOx and phosphate concentrations as high as 13.35 and 0.94 μmol/L, respectively, at 105 m on the shelf. Upwelled water properties matched slope water properties between 240 and 370 m, indicating water can be upwelled over depths of 200 m or more. Upwelling from these depths sources South Australian Basin Central Water of Southern Ocean origin, which is transported west along the slope by the Flinders Current System. New results for nutrients show average values of NOx and phosphate during months of strong upwelling to be 6.1 times and 4.6 times greater, respectively, than during winter months, and that upwelled water can have nutrient concentrations up to 90 times higher than those in summer surface waters. Strong relationships between temperature and nutrients on the slope can help estimate nutrient concentrations supplied to the shelf during upwelling events. Upwelled water was also low in silicate, a signature of Southern Ocean water masses, which has implications for phytoplankton community structure and diatom abundance on the shelf.

Comparative Study on Structure of Exopolysaccharide and Capsular Polysaccharide Produced by Southern Ocean Origin Pseudoalteromonas sp. MB-16

A comparative study on the basic structural characteristics of exopolysaccharide (EPS) and capsular polysaccharide (CPS) produced by the Southern Ocean (Indian Sector) origin Pseudoalteromonas sp. MB-16 has been conducted. It has a higher production rate of EPS as compared to that of CPS and the maximum production rate is 0.7 g/l when glucose is the carbon source. Elemental analysis and infrared spectroscopic study have given a brief compositional insight on the EPS and CPS produced by the marine bacterium, whereas scanning electron micrographs have revealed the morphological structure of Pseudoalteromonas sp. derived EPS and CPS. Both the polysaccharides show significant thermal stability as confirmed by thermogravimetric analysis. In all the experiments, Pseudoalteromonas sp. derived EPS and CPS have given similar results when compared to standard bacterial polysaccharides dextran and guar gum. This present study gave an important preliminary idea about the nature of bacterial polysaccharides produced by the Southern Ocean (Indian Sector) origin microbe.

Southern Ocean Origin of Multidecadal Variability in the North Brazil Current

AbstractAnalysis of model data from a long (200 years) simulation of a high‐resolution version of the Parallel Ocean Program indicates a connection between a mode of multidecadal variability in the Southern Ocean, the so‐called Southern Ocean Mode, and multidecadal variability in the North Brazil Current. The multidecadal sea surface height variability in the Southern Ocean propagates northward and submerges at about 40°S. Northward propagating anomalies in ocean heat content are found between 5° and 40°S at depths down to 1 km and affect the North Brazil Current. Similar variability and connections between Southern Ocean and North Brazil Current are also found in a (200 years) simulation of a high‐resolution global version of the Community Earth System Model. The results provide a new mechanism for the low‐frequency variability of the North Brazil Current.

A global ocean climatology of preindustrial and modern ocean δ13C

AbstractWe present a global ocean climatology of dissolved inorganic carbon δ13C (‰) corrected for the 13C‐Suess effect, preindustrial δ13C. This was constructed by first using Olsen and Ninnemann's (2010) back‐calculation method on data from 25 World Ocean Circulation Experiment cruises to reconstruct the preindustrial δ13C on sections spanning all major oceans. Next, we developed five multilinear regression equations, one for each major ocean basin, which were applied on the World Ocean Atlas data to construct the climatology. This reveals the natural δ13C distribution in the global ocean. Compared to the modern distribution, the preindustrial δ13C spans a larger range of values. The maxima, of up to 1.8‰, occurs in the subtropical gyres of all basins, in the upper and intermediate waters of the North Atlantic, as well as in mode waters with a Southern Ocean origin. Particularly strong gradients occur at intermediate depths, revealing a strong potential for using δ13C as a tracer for changes in water mass geometry at these levels. Further, we identify a much tighter relationship between δ13C and apparent oxygen utilization (AOU) than between δ13C and phosphate. This arises because, in contrast to phosphate, AOU and δ13C are both partly reset when waters are ventilated in the Southern Ocean and underscore that δ13C is a highly robust proxy for past changes in ocean oxygen content and ocean ventilation. Our global preindustrial δ13C climatology is openly accessible and can be used, for example, for improved model evaluation and interpretation of sediment δ13C records.

Intermediate and deep water mass distribution in the Pacific during the Last Glacial Maximum inferred from oxygen and carbon stable isotopes

Intermediate ocean circulation changes during the last Glacial Maximum (LGM) in the North Pacific have been linked with Northern Hemisphere climate through air–sea interactions, although the extent and the source of the variability of the processes forcing these changes are still not well resolved. The ventilated volumes and ages in the upper wind driven layer are related to the wind stress curl and surface buoyancy fluxes at mid to high latitudes in the North Pacific. In contrast, the deeper thermohaline layers are more effectively ventilated by direct atmosphere-sea exchange during convective formation of Subantarctic Mode Waters (SAMW) and Antarctic Intermediate Waters (AAIW) in the Southern Ocean, the precursors of Pacific Intermediate Waters (PIW) in the North Pacific. Results reported here show a fundamental change in the carbon isotopic gradient between intermediate and deep waters during the LGM in the eastern North Pacific indicating a deepening of nutrient and carbon rich waters. These observations suggest changes in the source and nature of intermediate waters of Southern Ocean origin that feed PIW and enhanced ventilation processes in the North Pacific, further affecting paleoproductivity and export patters in this basin. Furthermore, oxygen isotopic results indicate these changes may have been accomplished in part by changes in circulation affecting the intermediate depths during the LGM.

Deglacial diatom productivity and surface ocean properties over the Bermuda Rise, northeast Sargasso Sea

Diatom assemblages document surface hydrographic changes over the Bermuda Rise. Between 19.2 and 14.5 ka, subtropical diatom species and Chaetoceros resting spores dominate the flora, as in North Atlantic productive regions today. From 16.9 to 14.6 ka, brackish and fresh water diatoms are common and their contribution is generally coupled with total diatom abundance. This same interval also contains rare grains of ice‐rafted debris. Coupling between those proxies suggests that successive discharge of icebergs might have stimulated productivity during Heinrich event 1 (H1). Iceberg migration to the subtropics likely created an isolated environment involving turbulent mixing, upwelled water, and nutrient‐rich meltwater, supporting diatom productivity in an otherwise oligotrophic setting. In addition, the occurrence of mode water eddies likely brought silica‐rich waters of Southern Ocean origin to the euphotic zone. The persistence of lower‐salinity surface water beyond the last ice rafting suggests continued injection of fresh water by cold‐core rings and advection around the subtropical gyre. These results indicate that opal productivity may have biased estimates of meridional overturning based on 231Pa/230Th ratios in Bermuda Rise sediments during H1.

Impacts of interruption of the Agulhas leakage on the tropical Atlantic in coupled ocean–atmosphere simulations

In this paper we use a coupled ocean–atmosphere model to investigate the impact of the interruption of Agulhas leakage of Indian ocean water on the tropical Atlantic, a region where strong coupled ocean–atmosphere interactions occur. The effect of a shut down of leakage of Indian ocean water is isolated from the effect of a collapse of the MOC. In our experiments, the ocean model is forced with boundary conditions in the southeastern corner of the domain that correspond to no interocean exchange of Indian ocean water into the Atlantic. The southern boundary condition is taken from the Levitus data and ensures an MOC in the Atlantic. Within this configuration, instead of warm and salty Indian ocean water temperature (cold) and salinity (fresh) anomalies of southern ocean origin propagate into the South Atlantic and eventually reach the equatorial region, mainly in the thermocline. This set up mimics the closure of the “warm water path” in favor of the “cold water path”. As part of the atmospheric response, there is a northward shift of the intertropical convergence zone (ITCZ). The changes in trade winds lead to reduced Ekman pumping in the equatorial region. This leads to a freshening and warming of the surface waters along the equator. Especially in the Cold Tongue region, the cold and fresh subsurface anomalies do not reach the surface due to the reduced upwelling. The anomaly signals are transported by the equatorial undercurrent and spread away from the equator within the thermocline. Part of the anomaly eventually reaches the Tropical North Atlantic, where it affects the Guinea Dome. Surprisingly, the main effect at the surface is small on the equator and relatively large at the Guinea Dome. In the atmosphere, the northward shift of the ITCZ is associated with a band of negative precipitation anomalies and higher salinities over the Tropical South Atlantic. An important implication of these results is that the modified water characteristics due to a shut down of the Agulhas leakage remain largely unaffected when crossing the equatorial Atlantic and therefore can affect the deepwater formation in the North Atlantic. This supports the hypothesis that the Agulhas leakage is an important source region for climate change and decadal variability of the Atlantic.

Evolution of the Deep and Bottom Waters of the Scotia Sea, Southern Ocean, during 1995–2005*

Abstract The Southern Ocean hosts the formation of the densest layers of the oceanic overturning circulation and provides a climatically sensitive element of deep ocean ventilation. An oceanographic section across the eastern Scotia Sea occupied in 1995, 1999, and 2005 reveals significant variability in the deep and bottom waters of Southern Ocean origin. Warming (∼0.1°C) of the warm midlayer waters in the Scotia Sea between 1995 and 1999 reversed through to 2005, reflecting changes seen earlier upstream in the Weddell Sea. The volume of deep waters with potential temperature less than 0°C decreased during 1995–2005, though such a reduction was only clear between 1995 and 1999 at the southern end of the section. The abyssal waters of the eastern Scotia Sea changed circulation between 1995 and 1999, with the dominant point of their entry to the basin shifting from the south to the northeast; by 2005, the former route had regained dominance. These changes are best explained by interannual variations in the deep waters exiting the Weddell Sea, superimposed on a longer-term (decadal) warming trend. The interannual variations are related to changes in the strength of the Weddell Gyre, reflecting large-scale atmospheric variability that may include the El Niño–Southern Oscillation phenomenon. The Scotia Sea is the most direct pathway for dense waters of the overturning circulation emanating from the Weddell Sea to fill much of the World Ocean abyss. The regional changes reported here have the potential to affect the climatically significant ventilation of the global ocean abyss.

Patterns of opal and radiolarian change in the Antarctic mid-Paleogene: clues to the origin of the Southern Ocean

The timing of the origin of the Southern Ocean is important for studies of Antarctic biologic evolution and for understanding past climate change. The long standing theory that separation of Australia from Antarctica at the end of the Eocene allowed the development of a circumpolar ocean circulation, isolating the Antarctic continent and causing ice-sheet growth at the Eocene-Oligocene boundary, has recently been challenged by ODP Leg 189 drilling south of Australia. Based on these new cores it has been proposed that separation and Southern Ocean formation had already occurred within the late Eocene, approximately 2 my before the Antarctic became glaciated. This proposal however extrapolates data from a limited area to the history of a large circumpolar ocean region. To better determine Southern Ocean history we have compiled data from a large number of locations around Antarctica on opal accumulation and have analyzed circum-polar radiolarian faunas for development of endemism and evolutionary turnover. Our results show that opal deposition was widespread in the late Eocene Antarctic oceans with concentrations similar to that of the early Oligocene; substantial endemism was already present in late Eocene radiolarian faunas, and that most evolutionary turnover, in particular the origin of taxa characteristic of the early Oligocene, had already taken place within the late Eocene (ca 35 Ma). We conclude that there is a signifi cant (ca 2 my) gap between a late Eocene Southern Ocean origin and later, Eocene-Oligocene boundary Antarctic glaciation.

A 350-ky radiolarian record off Lüderitz, Namibia—evidence for changes in the upwelling regime

The study of radiolarian assemblages from Core MD 962086 provides new information on the variability in the upwelling intensity and origin of upwelled water masses over the past 350 ky in one of the major filamentous regions of the Benguela Upwelling System (BUS), located off Lüderitz, Namibia. The use of key radiolarian species to trace the source of upwelled waters, and the use of a radiolarian-based upwelling index (URI) to reconstruct the upwelling intensity represent the first use of radiolarians for paleoceanographic reconstructions in the BUS. These radiolarian-based proxies indicate strongest upwelling during Marine Isotope Stages (MIS) 3, 5, and 8, which compares well with other studies. While during MIS 3 and 8, the radiolarian-based proxies indicate the influx of waters of Southern Ocean origin, they also point to the increased influence of tropical waters during the lower portion of MIS 5. During MIS 2, 4 and 6 the radiolarian assemblages indicate generally lower upwelling intensities, although this signal is complicated by the increased occurrence of organic carbon in the sediments during these intervals. During MIS 2 there appears to be less of an input of Southern Ocean waters to the BUS, although during the also glacial MIS 4 and 6, there is evidence for an increased influence of cold Antarctic waters. The comparison of the results from Core MD 962086 with other studies in the BUS area indicates a non-uniform pattern of upwelling intensity and advection of cold, southern waters into this system during MIS 2. Weaker upwelling signaled by the radiolarian-based proxy in MIS 4 is in contrast to other studies that indicate higher productivity during this time period. In general, the data show that there is a strong spatiotemporal complexity in upwelling intensity in the BUS and that the advection of water into it is not strongly tied to glacial–interglacial variations in climate.

Southern Ocean origin for the resumption of Atlantic thermohaline circulation during deglaciation.

During the two most recent deglaciations, the Southern Hemisphere warmed before Greenland. At the same time, the northern Atlantic Ocean was exposed to meltwater discharge, which is generally assumed to reduce the formation of North Atlantic Deep Water. Yet during deglaciation, the Atlantic thermohaline circulation became more vigorous, in the transition from a weak glacial to a strong interglacial mode. Here we use a three-dimensional ocean circulation model to investigate the impact of Southern Ocean warming and the associated sea-ice retreat on the Atlantic thermohaline circulation. We find that a gradual warming in the Southern Ocean during deglaciation induces an abrupt resumption of the interglacial mode of the thermohaline circulation, triggered by increased mass transport into the Atlantic Ocean via the warm (Indian Ocean) and cold (Pacific Ocean) water route. This effect prevails over the influence of meltwater discharge, which would oppose a strengthening of the thermohaline circulation. A Southern Ocean trigger for the transition into an interglacial mode of circulation provides a consistent picture of Southern and Northern hemispheric climate change at times of deglaciation, in agreement with the available proxy records.

Late Cretaceous to early Quaternary organic sedimentation in the eastern Equatorial Atlantic

Reconstructing the long-term evolution of organic sedimentation in the eastern Equatorial Atlantic (ODP Leg 159) provides information about the history of the climate/ocean system, sediment accumulation, and deposition of hydrocarbon-prone rocks. The recovery of a continuous, 1200 m long sequence at ODP Site 959 covering sediments from Albian (?) to the present day (about 120 Ma) makes this position a key location to study these aspects in a tropical oceanic setting. New high resolution carbon and pyrolysis records identify three main periods of enhanced organic carbon accumulation in the eastern tropical Atlantic, i.e. the late Cretaceous, the Eocene–Oligocene, and the Pliocene–Pleistocene. Formation of Upper Cretaceous black shales off West Africa was closely related to the tectono-sedimentary evolution of the semi-isolated Deep Ivorian Basin north of the Côte d’Ivoire–Ghana Transform Margin. Their deposition was confined to certain intervals of the last two Cretaceous anoxic events, the early Turonian OAE2 and the Coniacian–Santonian OAE3. Organic geochemical characteristics of laminated Coniacian–Santonian shales reveal peak organic carbon concentrations of up to 17% and kerogen type I/II organic matter, which qualify them as excellent hydrocarbon source rocks, similar to those reported from other marginal and deep sea basins. A middle to late Eocene high productivity period occurred off equatorial West Africa. Porcellanites deposited during that interval show enhanced total organic carbon (TOC) accumulation and a good hydrocarbon potential associated with oil-prone kerogen. Deposition of these TOC-rich beds was likely related to a reversal in the deep-water circulation in the adjacent Sierra Leone Basin. Accordingly, outflow of old deep waters of Southern Ocean origin from the Sierra Leone Basin into the northern Gulf of Guinea favored upwelling of nutrient-enriched waters and simultaneously enhanced the preservation potential of sedimentary organic matter along the West African continental margin. A pronounced cyclicity in the carbon record of Oligocene–lower Miocene diatomite–chalk interbeds indicates orbital forcing of paleoceanographic conditions in the eastern Equatorial Atlantic since the Oligocene–Miocene transition. A similar control may date back to the early Oligocene but has to be confirmed by further studies. Latest Miocene–early Pliocene organic carbon deposition was closely linked to the evolution of the African trade winds, continental upwelling in the eastern Equatorial Atlantic, ocean chemistry and eustatic sea level fluctuations. Reduction in carbonate carbon preservation associated with enhanced carbon dissolution is recorded in the uppermost Miocene (5.82–5.2 Ma) section and suggests that the latest Miocene carbon record of Site 959 documents the influence of corrosive deep waters which formed in response to the Messinian Salinity Crisis. Furthermore, sea level-related displacement of higher productive areas towards the West African shelf edge is indicated at 5.65, 5.6, 5.55, 5.2, 4.8 Ma. In view of humid conditions in tropical Africa and a strong West African monsoonal system around the Miocene–Pliocene transition, the onset of pronounced TOC cycles at about 5.6 Ma marks the first establishment of upwelling cycles in the northern Gulf of Guinea. An amplification in organic carbon deposition at 3.3 Ma and 2.45 Ma links organic sedimentation in the tropical eastern Equatorial Atlantic to the main steps of northern hemisphere glaciation and testifies to the late Pliocene transition from humid to arid conditions in central and western African climate. Aridification of central Africa around 2.8 Ma is not clearly recorded at Site 959. However, decreased and highly fluctuating carbonate carbon concentrations are observed from 2.85 Ma on that may relate to enhanced terrigenous (eolian) dilution from Africa.

The Southerly Burster of South Eastern Australia: An Orographically Forced Cold Front

Abstract This paper presents an observational study of the “southerly burster”, or “southerly buster”, a particularly intense type of orographically and thermally influenced cold front which occurs in spring and summer along the southern coast of New South Wales in eastern Australia. It may have counterparts elsewhere. A brief review is given of the synoptic aspects of seventeen such fronts which occurred during the period January 1972 to January 1978. These had diverse origins, only rarely being fronts of Southern Ocean origin; mostly they developed ahead of Southern Ocean fronts. In all cases orographical effects appear to play an important role in genesis. A more detailed study is made of two southerly bursters which occurred during an observational program held from 21 November 1982 until 13 December 1982, providing the synoptic background for a more detailed analysis of the structure of these phenomena in another paper. The first was particularly severe, producing wind gusts of up to 24.7 m s−1 and t...