Earth’s climate is influenced by a number of factors and their interaction. Ocean currents transporting huge water masses have a strong influence on climate. In this way heat is transported from the equators towards the poles. The effect is documented e.g. in the western part of Ireland and the UK where branches of the gulf stream, named after the Gulf of Mexico, lead to a mild climate and allow the growth of palm trees. In the same latitudes in North America these plants can only be found in well heated buildings. The difference results from the heating system of the gulf streams, which provides the equivalent of 30,000 million tons of coal produced warmth showing how well a current transports heat.
Why do we need to understand the development of the Southern Ocean's gateways?
Additionally to the primary importance of oceanic currents on climate are their pathways. These are not arbitrary. Submarine mountains, ridges and barriers steer their paths. The location of the continents further influences the pathways of the oceanic circulation. A simple look onto a globe tells everyone that there is no direct circulation between the Pacific Ocean and the Caribbean. Panama prohibits this. But until about 6 million years ago North and South America were not connected via this land bridge and water could pass freely from the Pacific Ocean into the Atlantic at this location. With the relocation of both continents this gateway was closed thus interrupting the water mass exchange.
The displacement of the continents, i.e. the plate tectonic change in their geographic location, hence enforces modifications in pathways of oceanic currents. The plate tectonic development of Earth thus directly affects climate. In order to gain knowledge on the evolution of Erath’s climate we reconstruct the plate tectonic development of Earth thus contributing to a better understanding of the presently active processes on Earth and their impact on climate.
South African gateway
Reconstruction of the South African gateway (146 Ma, 135 Ma, 127 Ma, 120 Ma from König and Jokat, 2010, 100 Ma from Gohl et al., 2011) showing the development of the Mozambique Ridge (MozR) and the Agulhas Plateau-North East Georgia Rise-Maud Rise (AP-NEGR-MR) Large Igneous Province. The yellow arrows show possible surface (dashed lines) and deep (solid lines) circulation (Uenzelmann-Neben, 2013).
Development of the South African deep water gateway and water mass exchange
The global thermohaline circulation is strongly affected by the water mass exchange between the Indian Ocean and the South Atlantic Ocean. In this way, the water mass exchange has a strong influence on our climate.
Only a deep and wide gateway south of South Africa has allowed the necessary water mass exchange. Today we observe that the path of the Agulhas Current is controlled by the shelf edge, while the paths of Agulhas Rings are strongly controlled by the topography of the Agulhas Ridge. Agulhas Current and Rings enable the transfer of energy and heat between the Indian and the Atlantic Oceans. Elevations in seafloor such as Mozambique Ridge and Agulhas Plateau constitute obstacles for the paths of the oceanic circulation and lead to modifications in the paths. The formation of larger magmatic-structural units such as Large Igneous Provinces (LIP) results in constrictions in a gateway and hence a hindrance/interruption of the water mass exchange.
Several hypotheses suggest the formation of a LIP for the South African gateway for different periods and extents: a) a South Africa giant LIP formed between 140 Ma and 95 Ma, and b) in form of two LIPs created 140-120 Ma (Mozambique Ridge) and 105-95 Ma (Agulhas Plateau). The second hypothesis would allow a deep water circulation between Tethys and the opening Southern Ocean already at 120 Ma. Based on plate tectonic reconstructions Uenzelmann-Neben et al. show possible paths for both surface and deep circulation for different periods. This shows that both Mozambique Ridge and Agulhas Plateau constitute possible obstacles. Still, it has remained under discussion when and in which order these structures were formed and whether they have the same origin.
'Bright spots' have been identified in seismic reflection data southeast of South Africa and are interpreted as black shales 85-80 Ma. Black shales are deposited under anoxic condition and thus point towards a restricted circulation within this gateway at that period. An open circulation has been determined to have existed since Oligocene times. Since the Miocene the circulation appears to be comparable to the present one.
While we have derived relatively clear ideas on the development of the water mass exchange since the Neogene, we know little about the early phases of the circulation. This will be the focus of future studies, e.g. the BMBF project SLIP.
Drilling the younger history of water mass exchange!
The younger history of the water mass exchange and its impact on the development of humans is the focus of IODP Exp 361 Southern African Climates. Six sites along the southeastern and southwestern continental margin of South Africa as well as on the southern Agulhas Plateau will be drilled to gather information on the development of the so called Agulhas leakage. We have been involved in the development of the project and now participate in the drilling expedition.
This project has been funded by the Bundesministerium für Bildung, Forschung und Technologie under contracts No 03G0532A 'SETARAP', 03G0182A 'AISTEK-I', and 03G0232A 'SLIP'.
Effect of opening of Drake Passage on circulation in the South Atlantic
Sedimentary deposits in the Argentine Basin document modifications in circulation
Regional bathymetric map of the Argentine basin with the general circulation scheme of deep-water masses indicated. Legend for the water masses: ACC = Antarctic Circumpolar Current; AABW = Antarctic Bottom Water; LCDW = Lower Circumpolar Deep Water; UCDW = Upper Circumpolar Deep Water; and NADW = North Atlantic Deep Water. Legend for barriers (yellow) and passages (orange) of deep circulation: DP = Drake Passage; M/FE = Malvinas-Falkland Escarpment; M/FP = Malvinas-Falkland Passage; M/FR = Malvinas- Falkland Ridge; MEB = Maurice Ewing Bank; NGP = Northeast Georgia Passage; NGR = Northeast Georgia Ridge; SRP = Shag Rocks Passage; and IOR = Islas Orcadas Rise. Modified after Arhan et al. , Reid , and Gruetzner et al. . GCSD = giant canyons sector drift and GTSD = giant terraces sector drift. The seismic profiles used are shown as red lines. Major transfer zones [Franke et al., 2007] are shown as green dotted lines. Boreholes used for age assignments are indicated by black dots.
The opening of Drake Passage and the Scotia Sea enabled the exchange of water masses between the southern Pacific and the South Atlantic. In this way heat and energy could be transferred between the two oceans. Together with the opening of the Tasman Gateway this allowed the establishment of the Antarctic Circumpolar Current (ACC) thermally isolating Antarctica, which has been considered a one of the major causes for the onset of widespread glaciation. Both tectonic movements within Drake Passage and the Scotia Sea as well as modifications in climate have led to changes in intensity and pathway of the ACC and the water masses flowing within it. A study of the sediment drifts shaped by Circumpolar Deepwater, Weddell Sea Deepwater (WSDW) and Antarctic Bottomwater (AABW) using high-resolution seismic reflection data provides information on modifications of the circulation resulting from tectonic movements and changes in climate.
Chronological reconstruction of influence of circulation on deposits
The sedimentary record at the eastern continental margin of Argentina is strongly shaped by bottom water flow that originates in the Southern Ocean both in the ACC, WSDW, and AABW may contain an important palaeo-climate record that has yet to be sampled by ocean drilling. Our analysis of four Cenozoic time slices in a dense net of seismic reflection profiles provides basic information about variations in the relative importance of downslope versus along-slope depositional sediment transport processes at the central Argentine margin. For a detailed reconstruction of the evolutionary stages we identified seismic features diagnostic of current or gravity controlled sedimentation and mapped changes in depocentre location and sedimentation rates.
Paleocene and Eocene are characterized by pelagic sedimentation within a broad depocentre. For these time intervals of warm global climate there is little evidence for the influence of bottom current activity and/or gravitational sediment transport. During the Oligocene to the middle Miocene a giant elongated detached drift developed at the lower slope in a deep marine environment. Drift growth and major erosion at the adjacent continental slope point to a depositional regime that is strongly current dominated, likely by northward directed flow of Antarctic water masses (LCDW, AABW).
A major change in depositional style of the central Argentine margin occurred during the middle to late Miocene when the trough between the slope and the detached drift was filled with sediment. As a major depositional process we suggest gravity controlled downslope sediment transport possibly triggered by tectonic uplift. However, contouritic deposition may have also substantially contributed to the dispersal of the entrained sediment.
The most recent (Plio-Pleistocene) of the investigated time periods exhibits a variety of co-occurring seismic and morphological features that are diagnostic for both downslope (canyons, mass transport deposits) and along-slope (channels, sediment waves, plastered drifts) sediment redistribution. In general, current controlled sedimentation is more obvious south of 43.5 S while in the northern part of the working area contouritic features are heavily reshaped by cross-slope erosion through canyons.
The Agulhas Ridge - An obstacle for water mass exchange
The role of the Agulhas Ridge as a barrier to the exchange of water masses between high and low latitudes
The Agulhas Ridge forms an elongated part of the Agulhas-Falkland Fracture Zone (AFFZ) (43° S/9° E - 41° S/16° E). It rises more than 2,000 m above the surrounding seafloor. In the northeast the ridge is characterized by a plateau whereas the main ridge is built up by two parallel segments. The ridge segments are separated by a deep depression, which is filled with sediments of > 1000 m thickness. The inner flanks of the ridge segments are much steeper than the outer flanks. The ridge itself is of tectono-magmatic origin and shows only a very thin sedimentary cover. We have aimed to solve the following questions:
Has the Agulhas Ridge been reactivated tectono-magmatically?
Existing seismic reflection profiles show disturbances in the sedimentary layers. Basement highs in places pierce through the sedimentary column with basement being exposed at the seafloor. At least the pre-Oligocene sequences have been deformed, which points to a reactivation of the ridge in mid-Oligocene times. It is not clear what triggered the reactivation. Material channelized from the Discovery Hotspot via the AAFZ to the Agulhas Ridge has been discussed as an origin. To answer these questions we needed to map the distribution of the sedimentary layers and the spatial extent of the ridge using a grid of seismic reflection profiles and multibeam bathymetric data. This will lead to information on strike and structural relationship of the ridge segments to the AFFZ. Dredge locations were picked for a petrological sampling during MSM 19/3 based on this information.
The Agulhas Ridge as an obstacle for water masses exchange
We have aimed at a better understanding of both paths and intensity of the current system in the eastern South Atlantic during the Neogene. Constituting a topographic barrier the Agulhas Ridge has a strong influence on the exchange of water masses between high and lower latitudes and the inflow of both cold deep and warm, salty intermeadiate and surface water masses. Using high-resolution multichannel seismic reflection data acquired during RV Maria S. Merian cruise MSM 19/2 in October/November 2011 the palaeocirculation are studied. A correlation with results from ODP Leg 177 Sites 1088, 1089, and 1090 enables a reconstruction of the long-term evolution of the sedimentation processes and oceanographic conditions during the Neogene and Quaternay.
The questions addressed include the following:
- What is the impact of the Agulhas Ridge with respect to oceanic circulation? The Agulhas Ridge has prevented a direct N-S water mass exchange and hence has restricted energy and heat transfer since its formation ~83 Ma. Presently, an eastsetting flow (Antarctic Circumpolar Current ACC, South Atlantic Current SAC, Circumpolar Deepwater CDW) can be observed south of the ridge. North of the ridge a westsetting flow is prevailent. Sediment drifts have been formed parallel to and north of the Agulhas Ridge, which indicate the influence of a water mass similar to CDW starting in Oligocene times. The tight seismic grid collected during cruise MSM 19/2 allows a detailed mapping of the drift structures and depot centres. Via the shape and orientation of depot centres we will identify changes in paths and intensity of the active currents. The previously existing seismic data could tell little about the situation south of the Agulhas Ridge. The new seismic lines extend well into the northern Agulhas Basin and enable an identification as well as mapping of sediment drifts there. This way we are able to study the development of paths and intensity in the northern Agulhas Basin as well.
The transverse Agulhas ridge separates the Cape Basin from the Agulhas Basin and controls the exchange of water masses between these two basins. Small scale buried drifts, moats and sheet like deposits indicate that sedimentation was controlled by bottom currents since the late Eocene. After a pronounced early Oligocene erosional event resulting from the onset of Lower Circumpolar Deepwater (LCDW) flow, drift formation intensified. The type, position and formation history of the interpreted drifts suggest that the pathways of LCDW flow have undergone little change during the last ~33 Ma and followed roughly todays 4900 m depth contour. Northwest of the Cape Rise Seamount we found a mounded drift with an oval shape, a height of ~400 m and a width of ~50-60 km indicating a clockwise circulating bottom water gyre in that area. Extensive drifts in the Cape Basin occur as features confined between the Agulhas Ridge and Cape Rise seamounts and as mounded and sheeted drifts further to the West. The confined drifts show erosional features on both flanks suggesting a West setting bottom water flow along the northern flank of the Agulhas Ridge and an opposing eastward directed flow along the southern rim of the Cape rise seamount group. In contrast to the large drift deposits in the Cape Basin smaller, confined drifts showing more erosional features are found south of the Agulhas Ridge. Together these findings suggest that the deepest LCDW flowed anticlockwise around the Agulhas Ridge before taking a major clockwise loop in the Cape Basin. The returning bottom water then flowed around the Cape Rise seamounts before entering the Indian Ocean.
- The distribution of sedimentary layers on the eastern plateau of the ridge is largely unknown. This eastern plateau is of major importance, since is provides a kind of junction between the central and southern routes of Agulhas Rings. When can we identify the first traces of sediment transport in this area? Can we identify a depth interval for the influence of Agulhas Rings? The collected grid of seismic reflection profiles will lead to a better understanding on the role this topographic feature has had on the paths of Agulhas Rings. We will identify current induced structures such as sediment drifts, erosional horizons and scarps and via the age-depth model provided by ODP Site 1088 use them to conclude on the activity periods and paths of Agulhas Rings.
- The seismic data show a sedimentary column with at least 1 km thickness on the eastern plateau of the Agulhas Ridge, which are of mid-Miocene to recent age for the upper 240 m. Here, the ridge rises to a water depth of ~1,700 m and hence cannot receive turbidity currents of slides and slump masses. Age, origin and nature of the material are thus unclear. Do the sediments correspond to material observed on the Agulhas Bank or the Falkland Plateau? The newly collected seismic reflection data will enable to map the sedimentary structures and decipher the sedimentation history of the area.
Effect of opening of the Tasman Gateway on circulation in the South Pacific
Sedimentary structures east of New Zealand teach us about ACC development
The Thermohaline Circulation (THC) is directly linked to global climate. Changes in global climate affect the THC and vice versa. Sedimentary archives recording circulation patterns at key locations of the THC are of great interest for interdisciplinary research. These archives contain valuable information about past changes in oceanic circulation and associated climate.
One of these key regions is the Tasmanian Gateway between Antarctica and Australia. The Deep Western Boundary Current (DWBC), the main inflow of deep coldwater, enters the Pacific Ocean at this location. The Antarctic Circumpolar Current (ACC) also passes through the Tasmanian Gateway directly affecting the coldwater inflow of the DWBC. It is still under debate whether the opening of the Tasmanian Gateway and subsequent establishment of the ACC has been the major cause leading to thermal isolation and Antarctic Glaciation or whether changes in atmospheric carbon dioxide concentration triggered Antarctic Ice Sheet growth. No major tectonic processes such as opening of circum-Antarctic seaways occurred in the Southern Ocean during the Eocene, but at least a partial glaciation of East Antarctica hints towards cooling resulting from CO2 drawdown. Intensive cooling and sea ice enhance the deep water formation. Thus, cold climate conditions in an ice covered Antarctica caused intensified deep-water formation, which then formed specific sedimentary features in sedimentary archives. Such an archive is located directly downstream of the Tasmanian Gateway, where a vast amount of sediment is injected into the DWBC and ACC flow path. The sediments are supplied by the Eastern New Zealand Oceanic Sedimentary System (ENZOSS), a sediment recycling system.
The sedimentary archive of the Outer Bounty Trough has allowed a further reconstruction of the Cenozoic history of deep-water flow at the eastern edge of the New Zealand Microcontinent (Zealandia). Four observed drift bodies in the Outer Bounty Trough present a more complete picture of the influence of palaeoclimate and tectonic events on the (Proto-) DWBC. This includes first evidence for cold deep-water flows before the opening of the Tasmanian Gateway. Drift body 1 and drift body 2 below the Marshall Paraconformity indicate a pre- Oligocene deep circulation east of Zealandia. Build-up of drift body 1 started during the Palaeocene and presumably reflects creation of a deep seaway and current formed by the separation of Zealandia from Antarctica. This deep flow was probably modified by climate changes during the Eocene, initiating the formation of drift body 2. Timing of drift crest migration coincided with warming during the Mid Eocene Climate Optimum. The next drift crest migration (from drift body 2 to drift body 3) can be linked directly to tectonic processes. The opening of the Tasmanian Gateway has had a major impact of the whole ENZOSS region causing a wide spread erosional event associated with the establishment of the Antarctic Circumpolar Current (ACC). Global cooling along with the establishment of the West Antarctic Ice Sheet interrupted deposition of drift body 3. Deposition started again with the onset of the Pliocene (drift body 4, ~5 Ma). The gentle slope of the Outer Bounty Trough in contrast to the steep flanks of the Campbell Plateau slowed down the DWBC allowing deposition. The last drift crest migration can be attributed to an intensified ACC flow due to cooling during the early Pleistocene recorded in the Emerald and Tasmanian Basins. The intensification is also evident in the DWBC flow suggesting an indirect influence of the ACC on the DWBC.
Interpreted section of profile AWI-20110006 showing the regional relevant reflectors and seismic units A to D. Observed drift bodies (DB 1–DB 4) are coloured and marked with numbers as defined in the text. Circles with crosses indicate inferred flowcores of a (Proto-)DWBC and the numbered arrows showthe displacement of the cores with time. 1: Displacement during Palaeocene/Eocene Boundary, 2: displacement while initiation of the ACC after opening of Tasmanian Gateway, 3: displacement after intensification of deep currents around 10Ma, 4: displacement after arrival of first turbidites fromthe Bounty Channel; abbreviations: DB=drift body, w.a.a.=weak amplitude area (transitional zone).