Roberto A. Violante

Erosional and depositional contourite features at the transition between the western Scotia Sea and southern South Atlantic Ocean: links with regional water-mass circulation since the Middle Miocene

The aim of the present study was to characterise the morpho-sedimentary features and main stratigraphic stacking pattern off the Tierra del Fuego continental margin, the north-western sector of the Scotia Sea abyssal plain (Yaghan Basin) and the Malvinas/Falkland depression, based on single- and multi-channel seismic profiles. Distinct contourite features were identified within the sedimentary record from the Middle Miocene onwards. Each major drift developed in a water depth range coincident with a particular water mass, contourite terraces on top of some of these drifts being associated with interfaces between water masses. Two major palaeoceanographic changes were identified. One took place in the Middle Miocene with the onset of Antarctic Intermediate Water flow and the enhancement of Circumpolar Deep Water (CDW) flow, coevally with the onset of Weddell Sea Deep Water flow in the Scotia Sea. Another palaeoceanographic change occurred on the abyssal plain of the Yaghan Basin in the Late Miocene as a consequence of the onset of Southeast Pacific Deep Water flow and its complex interaction with the lower branch of the CDW. Interestingly, these two periods of change in bottom currents are coincident with regional tectonic episodes, as well as climate and Antarctic ice sheet oscillations. The results convincingly demonstrate that the identification of contourite features on the present-day seafloor and within the sedimentary record is the key for decoding the circulation of water masses in the past. Nevertheless, further detailed studies, especially the recovery of drill cores, are necessary to establish a more robust chronology of the evolutionary stages at the transition between the western Scotia Sea and the southern South Atlantic Ocean.

Chapter 6 The Argentine continental shelf: morphology, sediments, processes and evolution since the Last Glacial Maximum

Abstract The Argentine continental shelf is one of the largest and smoothest siliciclastic shelves in the world. Although it is largely emplaced in a passive continental margin, the southernmost regions are related to transcurrent and active margins respectively associated with the Malvinas Plateau and Scotia Arc. Sea-level fluctuations, sediment dynamics and climatic/oceanographic processes were the most important conditioning factors in the modelling of the shelf, with a minor influence from isostatic and tectonic factors that are more relevant in the southernmost regions. The shelf is shaped by diverse geomorphic features, among which the most significant are four sets of terraces genetically associated to sea-level stillstands during the post-glacial transgression; the final one occurred at around 11 ka and is associated with the Younger Dryas event. The Last Glacial Maximum (LGM) sedimentary sequence is composed of, on average, 5–15 m-thick terrigenous, siliciclastic, relict–palimpsest sands mainly sourced from the Andean region, with minor amounts of bioclast and gravels, resulting from the reworking of pre-transgressive coastal environments.

Principles of Paleoceanographic Reconstruction

This chapter introduces the principles of paleoceanographic reconstructions and proxy data, focusing on the concepts of climatic, biological, geological, geochemical as well as other large-scale proxies, tracers and records useful for such reconstructions in the western South Atlantic. Different proxies particularly useful for the Argentine margin, including physical and chemical properties of sediments, microfossils and geochemical and isotopic properties, are described.

Climate and Oceanographic Background

The regional climatic and oceanographic characteristics that regulate the climate-ocean system in the region are explained. The climatic system has different features depending on regional and local forcings, such as the Intertropical Convergence Zone, the South American Monsoon System, the Southern Hemisphere Westerlies, and the South Atlantic Convergence Zone. The hydrographic structure is dominated by different water-masses of Antarctic and Tropical origin, related to the major circulation system of the Western South Atlantic linked to the Atlantic Meridional Overturning Circulation. The main water-masses constituting the hydrographic structure are the Tropical Water-South Atlantic Central Water, the Antarctic Intermediate Water, the North Atlantic Deep Water and the Antarctic Bottom Water, being most of the Antarctic-sourced water-masses genetically related to the circulation of the Antarctic Circumpolar Current.

Morphosedimentary Configuration of the Argentina Continental Margin

The major regional features of the margin are coastal plains, shelf, slope and rise. These features show varied characteristics depending on their location on each type of margin. They contain diverse subordinated morphosedimentary features related to the prevailing genetic processes acting on each of them and shaping their relief. The shelf is shaped in terraces at increasing stepping depths toward offshore, they having been genetically related to fluctuations in the sea-level rise during the post-glacial transgression. The slope is also shaped in terraces, but in this case, these features are genetically associated to the development of alongslope complex systems of mixed depositional and erosive contouritic features formed at different depths, possibly related to highly energetic interfaces between adjacent water masses that constitute the thermohaline oceanic system. Gravitational, downslope processes generating turbidites, mass transport and debris flows deposits, most of them acting inside or close to submarine canyons, actively interact with the contouritic processes. The rise is partially formed by gravity-driven deposits at the base of the slope, although in the southern part of the margin the alongslope, contouritic processes are strong enough to shape the rise and imprinting it with particular current-driven features.

State of the Art in the Paleoceanographic Reconstructions at the Argentina Continental Margin

The characteristics of the Argentina Continental Margin, particularly depending on its key location in the climate and oceanographic global system, make it to have a high potential for paleoclimatic, paleoceanographic and paleoenvironmental reconstructions. An overview of the present knowledge of the matter, given by the application of a varied set of proxies and tracers in different regions of the margin, like the shelf and the slope, is detailed.

The Argentina Continental Margin

The Argentina Continental Margin, located in the Southwestern Atlantic Ocean, is inserted in a key region of the World Ocean due to its significance in the global oceanographic–climatic interaction. It is the only place where Antarcticand Equator-sourced water-masses interact in mid-latitudes with a net transport of meridional heat between the Southern Pole and the Equator. On the other hand, the geotectonic history of the region imprints it with significant geological characteristics. As a result, the climatically, oceanographically and physically driven sedimentary processes occurred in the region have originated particular and almost unique morphosedimentary features, which constitute complete records of the processes involved in its evolution. Those features contain different kinds of proxies, tracers and records (biological, geochemical, sedimentological, morphological and structural) which provide valuable quantitative and qualitative evidences for detailed paleoceanographic, paleoclimatic and paleoenvironmental reconstructions. Therefore, the Argentine margin potentially behaves as a complete archive for understanding most of the unique oceanographic and climatic characteristics that occur in the region and impact int he rest of the Southern Hemisphere.

The Argentina Continental Margin: Location and Significance

The Argentina Continental Margin (ACM) is one of the largest margins on the Earth. It is located in a key region of the World Ocean that is highly significant in the planet’s oceanographic-climatic system. The evolution of the Argentine margin is explained in terms of the combination and interaction among geotectonic, oceanographic and climatic factors. Because of that evolution, four types of margins develop in the region: a passive-volcanic rifted, a transcurrent-sheared, a mixed and an active margin. These aspects determine particular tectonic and stratigraphic characteristics for each of them.

Continental Margins in the Global Context

As highly significant features of major order at the lands–oceans edge, continental margins play an exceptional role in the Earth system. They are relevant for understanding the continental drift and the birth of oceans, the endogenous and exogenous processes that regulate the planet evolution, the present and past climatic and oceanographic changes, and the carbon cycle and biogeochemistry of the Earth. On this basis, the importance of those features at a global scale is discussed. This chapter also provides the basic definitions needed for understanding the concept of continental margins.