Notes on Southern Ocean hydrography, sea-ice and bottom water formation

doi:10.1016/0031-0182(88)90119-8

Palaeogeography, Palaeoclimatology, Palaeoecology

Volume 67, Issues 1–2, September 1988, Pages 3-17

https://doi.org/10.1016/0031-0182(88)90119-8 Get rights and content

Abstract

The Southern Ocean circulation and sea-ice distribution is briefly described. The formation of extremely cold bottom water in the Weddell Sea and its relation to the floating Ronne-Filchner Ice Shelves is discussed. It is shown that a concentrated swift eroding bottom current with anomalous low $δ^{18} O$ ratio transports the cold and dense ice Shelf Water from the shelf towards large depths. Comments are made on possible implications of this process for the large-scale deep-water circulation and for the interpretation of sediment cores.

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Cited by (139)

Geomorphology of Ona Basin, southwestern Scotia Sea (Antarctica): Decoding the spatial variability of bottom-current pathways
2020, Marine Geology
Citation Excerpt :
The CDW is the deeper part of the ACC. It flows clockwise around the Antarctic continent and is the principal contributor to the boundary currents of the South Atlantic, South Pacific, and Indian oceans (Nowlin and Nowlin and Klinck, 1986; Foldvik and Gammelrsød, 1988; Naveira Garabato et al., 2002a). The boundaries of different ACC jets are composed of four deep-reaching hydrographic fronts (Orsi et al., 1995) that are strongly influenced by the Scotia Sea topography (Tarakanov, 2012) (Fig. 1B).
Ona Basin, the westernmost oceanic basin in the southern Scotia Sea, is affected by the opposite flows of Circumpolar Deep Water (CDW) and Weddell Sea Deep Water (WSDW); thus, it represents a key location for exploring seafloor morphologies influenced by bottom currents. The present study aims to capture the spatial arrangement of recent subsurface contourite features, assuming a latitudinal influence of water masses and the interactions between along- and downslope processes, in order to contribute to the knowledge of regional deepwater flow pathways and to the sedimentary model of small sediment-starved oceanic basins. To this end, the investigation combines an interpretation of multibeam bathymetry and parametric echo sounder seismic data complemented with hydrological data. The distribution of morpho-sedimentary features in Ona Basin reveals two major domains. The southern margin of the basin can be regarded as a mixed/hybrid system containing abundant sediment drifts with channels and contourite moats and a lateral continuity interrupted by downslope morphologies. In contrast, the northern abyssal setting comprises relatively homogeneous large sheeted drifts with superimposed sediment waves, mounded drifts, and several scattered erosive features, likely reflecting the more distinct influence of deepwater contourite processes. Our work demonstrates that tectonic features in the southern basin control the interaction between deepwater along- and downslope processes, as the westward flow of the WSDW is deflected, channelized, and intensified along its westward route. In the northern region, the study indicates an overall clockwise rotation of the WSDW flow, with the spatial and vertical variability of CDW and WSDW affecting the distribution of bottom-current features around seamounts and/or structural highs. The results underscore the importance of sloping interphases in the water mass vertical structure, the degree of basin confinement, and the influence of local bathymetric elevations in sedimentation models of small sediment-starved oceanic basins.
Precursors of Antarctic Bottom Water formed on the continental shelf off Larsen Ice Shelf
2015, Deep-Sea Research Part I: Oceanographic Research Papers
Citation Excerpt :
This water mass is formed by the mixing of Warm Deep Water (WDW) with Weddell Sea Bottom Water (WSBW) or with dense shelf waters. The continental shelf in front of Filchner-Ronne Ice Shelf (FRIS) is described as the main region where WSDW and WSBW are formed (Foldvik and Gammelsrød, 1988; Foldvik et al., 2004; Nicholls et al., 2009). Nevertheless, there is multiple evidence that the Larsen Ice Shelf (LIS) area also contributes, at least intermittently, to WSDW (Gordon et al., 1993, 2001; Fahrbach et al., 1995; Gordon, 1998; Schröder et al., 2002; Nicholls et al., 2004; Absy et al., 2008; Huhn et al., 2008; Jullion et al., 2013).
The dense water flowing out from the Weddell Sea significantly contributes to Antarctic Bottom Water (AABW) and plays an important role in the Meridional Overturning Circulation. The relative importance of the two major source regions, the continental shelves in front of Filchner-Ronne Ice Shelf and Larsen Ice Shelf, however, remains unclear. Several studies focused on the contribution of the Filchner-Ronne Ice Shelf region for the deep and bottom water production within the Weddell Gyre, but the role of the Larsen Ice Shelf region for this process, especially the formation of deep water, remains speculative. Measurements made during the Polarstern cruise ANT XXIX-3 (2013) add evidence to the importance of the source in the western Weddell Sea. Using Optimum Multiparameter analysis we show that the dense water found on the continental shelf in front of the former Larsen A and B together with a very dense water originating from Larsen C increases the thickness and changes the θ/S characteristics of the layer that leaves the Weddell Sea to contribute to AABW.
The global relevance of the Scotia Arc: An introduction
2015, Global and Planetary Change
Citation Excerpt :
The ACC flows clockwise around Antarctica north of the Circum-Antarctic Low Pressure Belt (Fig. 2). The ACC controls the transport of heat, salt, and nutrients in the Southern Ocean and is the principal contributor to the boundary currents of the South Atlantic, South Pacific and Indian oceans (Nowlin and Klinck, 1986; Foldvik and Gammelrsød, 1988; Naveira Garabato et al., 2002). The ACC also plays an important role in maintaining the thermal isolation of the continent.
The Scotia Arc, situated between South America and Antarctica, is one of the Earth's most important ocean gateways and former land bridges. Understanding its structure and development is critical for the knowledge of tectonic, paleoenvironmental and biological processes in the southern oceans and Antarctica. It extends from the Drake Passage in the west, where the Shackleton Fracture Zone forms a prominent, but discontinuous, bathymetric ridge between the southern South American continent and the northern tip of the Antarctic Peninsula to the active intra-oceanic volcanic arc forming the South Sandwich Island in the east. The tectonic arc comprises the NSR to the north and to the south the South Scotia Ridge, both transcurrent plate margins that respectively include the South Georgia and South Orkney microcontinents. The Scotia and Sandwich tectonic plates form the major basin within these margins. As the basins opened, formation of first shallow sea ways and then deep ocean connections controlled the initiation and development of the Antarctic Circumpolar Current, which is widely thought to have been important in providing the climatic conditions for formation of the polar ice-sheets. The evolution of the Scotia Arc is therefore of global palaeoclimatic significance. The Scotia Arc has been the focus of increasing international research interest. Many recent studies have stressed the links and interactions between the solid Earth, oceanographic, paleoenvironmental and biological processes in the area.
This special issue presents new works that summarize significant recent research results and synthesize the current state of knowledge for the Scotia Arc.
Oceanographic variability on the West Antarctic Peninsula during the Holocene and the influence of upper circumpolar deep water
2015, Quaternary Science Reviews
Recent intensification of wind-driven upwelling of warm upper circumpolar deep water (UCDW) has been linked to accelerated melting of West Antarctic ice shelves and glaciers. To better assess the long term relationship between UCDW upwelling and the stability of the West Antarctic Ice Sheet, we present a multi-proxy reconstruction of surface and bottom water conditions in Marguerite Bay, West Antarctic Peninsula (WAP), through the Holocene. A combination of sedimentological, diatom and foraminiferal records are, for the first time, presented together to infer a decline in UCDW influence within Marguerite Bay through the early to mid Holocene and the dominance of cyclic forcing in the late Holocene. Extensive glacial melt, limited sea ice and enhanced primary productivity between 9.7 and 7.0 ka BP is considered to be most consistent with persistent incursions of UCDW through Marguerite Trough. From 7.0 ka BP sea ice seasons increased and productivity decreased, suggesting that UCDW influence within Marguerite Bay waned, coincident with the equatorward migration of the Southern Hemisphere Westerly Winds (SWW). UCDW influence continued through the mid Holocene, and by 4.2 ka BP lengthy sea ice seasons persisted within Marguerite Bay. Intermittent melting and reforming of this sea ice within the late Holocene may be indicative of episodic incursions of UCDW into Marguerite Bay during this period. The cyclical changes in the oceanography within Marguerite Bay during the late Holocene is consistent with enhanced sensitively to ENSO forcing as opposed to the SWW-forcing that appears to have dominated the early to mid Holocene. Current measurements of the oceanography of the WAP continental shelf suggest that the system has now returned to the early Holocene-like oceanographic configuration reported here, which in both cases has been associated with rapid deglaciation.
A model of oceanic development by ridge jumping: Opening of the Scotia Sea
2014, Global and Planetary Change
Ona Basin is a small intra-oceanic basin located in the southwestern corner of the Scotia Sea. This region is crucial for an understanding of the early phases of opening of Drake Passage, since it may contain the oldest oceanic crust of the entire western Scotia Sea, where conflicting age differences from Eocene to Oligocene have been proposed to date. The precise timing of the gateway opening between the Pacific and Atlantic oceans, moreover, has significant paleoceanographic and global implications. Two sub-basins are identified in this region, the eastern and western Ona basins, separated by the submarine relief of the Ona High. A dense geophysical data set collected during the last two decades is analyzed here. The data include multichannel seismic reflection profiles, and magnetic and gravimetric data.
The oceanic basement is highly deformed by normal, reverse and transcurrent faults, as well as affected by deep intrusions from the mantle. The initial extension and continental thinning, with subsequent oceanic spreading, were followed by compression and thrusting. Several elongated troughs, bounded by faults, depict a thick sequence of depositional units in the basin. Eight seismic units are identified in a deep trough of the eastern Ona Basin. The deposits reach a thickness of 5 km, a consistent value not previously reported from the Scotia Sea. A body of chaotic seismic facies is also observed above the thinned continental crust of the Ona High. Magnetic seafloor anomalies older than C10 (~ 28.5 Ma) may be present in the region. The anomalies could include up to chron C12r (~ 32 Ma), although their identification is difficult, since the amplitude is subdued and the original oceanic crust was highly deformed by later faulting and thrusting. The magnetic anomaly distribution is not congruent with seafloor spreading from a single ridge. The basin plain is tilted and subducted southwestward below the South Shetland Islands Block, particularly in the western part, where an accretionary prism is identified. Such tectonics, locally affecting up to the most recent deposits, imply that a portion of the primitive oceanic crust is absent. Based on the stratigraphy of the deposits and the magnetic anomalies, an age of 44 Ma is postulated for the initiation of oceanic spreading in the eastern Ona basin, while spreading in the western Ona Basin would have occurred during the early Oligocene.
The tectonics, depositional units and the age of the oceanic crust provide additional evidence regarding the Eocene opening of Drake Passage. The initial tectonic fragmentation of the South America–Antarctic Bridge, followed by oceanic spreading, was characterized by jumping of the spreading centers. An Eocene spreading center in the eastern Ona Basin was the precursor of the Scotia Sea. A model comprising four tectonic evolutionary phases is proposed: Phase I, Pacific subduction — Paleocene to middle Eocene; Phase II, eastern Ona back-arc spreading — middle to late Eocene; Phase III, ridge jumping and western Ona back-arc spreading — early Oligocene; and Phase IV, ridge jumping and West Scotia Ridge spreading — early Oligocene to late Miocene.
The development of shallow gateways allowed for an initial connection between the Pacific and Atlantic oceans and, hence, initiated the thermal isolation of Antarctica during the middle and late Eocene. Deep gateways that enhanced the full isolation of Antarctica developed in Drake Passage from the Eocene/Oligocene transition onward. A significant correlation is observed between the tectonics, stratigraphic units and major climate events, thereby indicating the influence of the local tectonic and paleoceanographic events of the Southern Ocean on global evolution.
Tectonics and palaeoceanographic evolution recorded by contourite features in southern Drake Passage (Antarctica)
2013, Marine Geology
The sedimentary record in the vicinity of the triple junction at the southern Drake Passage is analyzed in order to decode the palaeoceanographic evolution and the influence of tectonic events. The break-up of the last connection between South America and Antarctica led to the circulation of important oceanographic bottom flows, including the Antarctic Circumpolar Current (ACC) and the Weddell Sea Deep Water (WSDW). The Shackleton Fracture Zone (SFZ), a ridge crossing the central Drake Passage, has been proposed as a major barrier that constrained the free circulation of bottom flows in the area, but whose timing and importance is poorly established. Also, the South Scotia Ridge (SSR), a prominent relief composing the southern part of the Scotia Arc, has controlled oceanographic exchanges between the Weddell and Scotia seas, as bottom flows from the Weddell Sea to the Scotia Sea have been conducted across narrow gateways along the SSR. On the basis of a network of multichannel seismic profiles, we interpret the uplift dynamics of the SFZ in the southern Drake Passage and its influence on the evolution of the bottom-current circulation and by extension on contourite processes.
Six main seismic units, identified and correlated between the Scotia and the Former Phoenix plates, depict a south-west-directed tilting of the deposits above a mid-Miocene reflector (Reflector c) on the Phoenix Plate. The regional correlation of the main reflections and contourite features indicates that the deepest fraction of the ACC, the Lower Circumpolar Deep Water (LCDW as the lower part of the Circumpolar Deep Water, CDW), flowed freely from the latest Eocene to the middle Miocene, and that these palaeo-flows have been active until the present in the abyssal plain of the central–western Scotia Sea. SFZ uplift was initiated, at the latest, during the middle Miocene (about 12 Ma), when the SFZ began to be an effective barrier to bottom flows in the southern Drake Passage. The ridge forced the ACC and the Polar Front to shift northward contributing to the thermal isolation of Antarctica and more polar conditions. The northward displacement of the LCDW and the opening of passages along the SSR favored the insertion of WSDW flows along the southern part of the Drake Passage, westward into the Pacific Ocean and northward into the abyssal plain of the southwestern Scotia Sea. Based on the morphology and evolution of the main erosional features, we also calculate a ratio between two volumetric flow rates related to the two major branches of the WSDW in the southwestern Scotia Sea area. The highest ratio is found for the age of Reflector b and is probably related with the strongest incursions of the WSDW in the area, as well as with SFZ uplift. This work demonstrates the common occurrence of large depositional and erosional features in deep marine environments related to bottom-current activity and their important implications on decoding palaeoceanographic, climatic and tectonic events.

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Notes on Southern Ocean hydrography, sea-ice and bottom water formation

Abstract

Deep-Sea Res.

Deep-Sea Res.

Deep-Sea Res.

Deep-Sea Res.

Deep-Sea Res.

Deep-Sea Res.

Die ozeanographischen Arbeiten der Deutschen Antarktischen Expedition 1911–1912

Arch. Dtsch. Seewarte

The hydrology of the Southern Ocean

Discovery Rep.