Agulhas Ridge Research Articles

Paired X‐Ray Micro CT Scanning and Individual Foraminifera Isotopic Analysis Reveal (De)coupled Changes in Carbonate Preservation and Temperature

AbstractThe composition and preservation state of biogenic carbonate archives, such as foraminiferal tests, record ocean chemistry during the lifetime of the organism and post‐depositional changes in ambient conditions via carbonate compensation. Depending upon the specific paleoclimate proxy, post‐depositional processes, including dissolution, may alter original paleoenvironmental signals captured by the foraminifer's test composition. Accordingly, quantifying dissolution independent of geochemical measurements can improve proxy interpretation. Developing independent tools may also be useful for investigating whether changes in paleoclimatic conditions are associated with changes in seawater carbonate chemistry. Such approaches can be improved further if they are applied to individual foraminiferal tests, as specimen‐to‐specimen differences can record higher‐frequency environmental changes compared to conventional bulk‐scale analyses. Here, we combine individual foraminiferal carbon and oxygen isotopic analyses (IFA) with X‐ray MicroCT Scanning to generate paired analyses of test density (a proxy for the extent of post‐depositional dissolution) and isotopic composition. As a proof‐of‐concept application of this approach, we analyze Globigerina bulloides tests from both coretop and latest Miocene/earliest Pliocene‐aged sediment from Ocean Drilling Project (ODP) Site 1088 (Agulhas Ridge). Our measurements and mixing model calculations show that within‐population differences in carbon and oxygen isotopic ratios are largely independent of dissolution extent. By comparing population averages from coretop and downcore sediments, we find that lower oxygen isotopic ratios (likely driven by higher calcification temperatures) are associated with greater extents of dissolution at ODP Site 1088. We interpret this finding to reflect coupled changes in carbonate chemistry and climatic conditions over million‐year timescales.

Biogenic silica accumulation and diatom assemblage variations through the Eocene-Oligocene Transition: A Southern Indian Ocean versus South Atlantic perspective

It is widely interpreted that there is a significant link between climate cooling at the Eocene-Oligocene Transition (EOT; ∼34 Ma) and subsequent diatom proliferation in the world's oceans during the mid to late Cenozoic. Yet, our understanding of biogenic silica flux through the EOT interval is based on limited data from a few sites, and there are many complications in making a meaningful comparison based on biogenic silica concentration data generated using different techniques. Here, we present new biogenic opal flux and diatom assemblage records across EOT from Ocean Drilling Program Site 748 (Southern Kerguelen Plateau, southern Indian Ocean), in addition to new biogenic opal flux records from the South Atlantic (Deep Sea Drilling Project Site 511, Falkland Plateau and Ocean Drilling Program Site 1090, Agulhas Ridge). Observed opal flux shifts and variability at Site 748 are consistent with published data from nearby Site 744; both sites show considerable shifts in biogenic opal accumulation rates corresponding to shifts in published benthic oxygen isotope records through the EOT. In contrast, our new opal flux record for Site 511, derived from biogenic opal concentration measurements via spectrophotometry, differs from the published record derived from insoluble residues, whereas new data generated for Site 1090 are generally consistent with previously published flux reconstructions. The South Atlantic biogenic opal flux records, however, are dissimilar from one another, and both are dissimilar from the Southern Indian Ocean records. Also, the taxonomic composition of the diatom assemblages from Sites 511, 748 and 1090 display considerable differences, with hemiauloids and rhizosolenids – generally inferred as indicative of oligotrophic conditions – being the dominant diatoms at the Southern Kerguelen Plateau (Site 748). Published records show that hemiauloid taxa are absent from the earliest Oligocene interval at Site 1090, which is widely seen as a record of deposition from nutrient-rich waters sustaining abundant diatom assemblages. These differences in diatom assemblages testify to regional differences in nutrient concentrations. In particular, the timing of biogenic opal flux pulses between Sites 1090 and 748 imply a shift in the locus of opal deposition to areas further south in the Southern Ocean across the EOT, likely related to proto-ACC development and strengthening of frontal boundaries. Thus, combined geochemical and micropaleontological evidence points to a regionally variable rather than a global, unified opal flux response to climate cooling through the Eocene-Oligocene Transition.

Physical Modelling of the Formation of the Meteor and Islas Orcadas Rises (South Atlantic)

The kinematic reorganization of the plate boundaries in the southeastern part of the Antarctic part of the South Atlantic, which expressed in the jump of the spreading axis of the Agulhas Ridge, has led to a restructuring of the structural plan of the region. This process resulted in the formation of the southern segment of the Mid-Atlantic Ridge (MAR), the extinction of the previously active Agulhas spreading ridge, and the formation of the Meteor and Islas Orcadas rises, marking the location of the MAR and located symmetrically about its axis. Based on the research results, the conditions for jumping the spreading axis were identified and an experimental model was constructed for the formation of accompanying structures, in which an important role was played by the accretion of the oceanic crust on the Agulhas Ridge and the westward migration of the Falkland Plateau. This resulted in the southward movement of the southern segment of the MAR, the formation of the Malvinas microplate, and a jump in the axis of the Agulhas spreading ridge, which led to the cessation of spreading on this ridge. An important role in the process of this kinematic restructuring was played by the activity of the Sean hotspot.

Tectonic dismemberment of Shona Hotspot volcano: Insights from Sites 698, 699, 701, and 703 bulk-rock Sr-Nd-Pb-Hf isotopic geochemistry

A genetic link between the Georgia-Islas Orcadas and Meteor guyots on opposite sides of the South Atlantic was proposed based on seafloor morphology and tectonic reconstructions. Here we test this model with geochemistry. We present new major and trace element and Sr, Nd, Pb, Hf radiogenic isotope data from volcanic and sedimentary rocks from Meteor Rise (ODP Site 703) and Georgia-Islas Orcadas Rise (ODP Sites 698, 699, and 701). Site 701 samples have slightly depleted isotopic compositions similar to southern Mid-Atlantic Ridge (SMAR) basalts, representing the upper mantle. Site 698, 699 and 703 volcanic rocks have initial Sr-Nd-Pb-Hf isotope ratios ((87Sr/86Sr)i = 0.70278–0.70660; (143Nd/144Nd)i = 0.51217–0.51283; (206Pb/204Pb)i = 18.09–18.75; (176Hf/177Hf)i = 0.28246–0.28299), extending from the Shona Hotspot track field (consisting of Richardson guyot, Agulhas Ridge and Meteor guyot samples) towards Proterozoic continental crust. These data support derivation from a common mantle plume, having formed contemporaneously as a single continuous hotspot track, which was subsequently divided by a ridge jump. Modelling of the isotope data show that binary mixing between an average Shona anomaly composition and an upper continental crustal component with a composition similar to the Site 698 sandstones can explain the data by 10–30% upper crustal assimilation to generate the composition of the Site 698, 699 and 703 volcanic rocks. Therefore, our geochemical data confirm the dismemberment of the Shona Hotspot track by a ridge jump and subsequent seafloor spreading.

Late middle Eocene to early Oligocene radiolarian biostratigraphy in the Southern Ocean (Agulhas Ridge, ODP Leg 117, Site 1090)

The analyses of abundant and well-preserved late middle Eocene to early Oligocene radiolarian assemblages were carried out on ODP Site 1090 (Leg 177) sediments in the Atlantic Sector of the Southern Ocean. A total of 83 radiolarian events are identified and correlated with the geochronologic time scale and paleomagnetic polarity data. The high latitude Eucyrtidium spinosum and Eucyrtidium antiquum Interval Zones are recognized in the studied interval at Hole 1090B, and three new subzones are proposed herein for the Eucyrtidium spinosum Zone: Eucyrtidium nishimurae Interval Subzone (Subchrons C17n.2r to C16r), Dendrospyris stabilis Interval Subzone (Subchrons C16n.2n to C13r), and Dicolocapsa microcephala Interval Subzone (Subchron C13r). The bases of these subzones are defined by the first occurrences of Eucyrtidium spinosum, Dendrospyris stabilis and Dicolocapsa microcephala, respectively. The proposal of these subzones could significantly improve the biostratigraphic resolution of the middle to late Eocene interval from high latitude regions. The FOs of Axoprunum irregularis and Eucyrtidium antiquum are nearly synchronous in the Southern Ocean, and could be used to estimate the Eocene/Oligocene boundary. Other radiolarian events such as the FOs of Dendrospyris stabilis, Dicolocapsa microcephala, and Zygocircus bütschli, and the LO of Eucyrtidium nishimurae are synchronous or approximately synchronous in the Southern Ocean. In contrast, the LOs of Axoprunum pierinae, Periphaena decora, and Siphocampe quadrata, and the FO of Artostrobus annulatus are commonly diachronous among different sites in the Southern Ocean.

Glacial reduction of AMOC strength and long-term transition in weathering inputs into the Southern Ocean since the mid-Miocene: Evidence from radiogenic Nd and Hf isotopes

Combined seawater radiogenic hafnium (Hf) and neodymium (Nd) isotope compositions were extracted from bulk sediment leachates and foraminifera of Site 1088, Ocean Drilling Program Leg 177, 2082 m water depth on the Agulhas Ridge. The new data provide a continuous reconstruction of long- and short-term changes in ocean circulation and continental weathering inputs since the mid-Miocene. Due to its intermediate water depth, the sediments of this core sensitively recorded changes in admixture of North Atlantic Deep Water to the Antarctic Circumpolar Current as a function of the strength of the Atlantic Meridional Overturning Circulation (AMOC). Nd isotope compositions (εNd) range from −7 to −11 with glacial values generally 1 to 3 units more radiogenic than during the interglacials of the Quaternary. The data reveal episodes of significantly increased AMOC strength during late Miocene and Pliocene warm periods, whereas peak radiogenic εNd values mark a strongly diminished AMOC during the major intensification of Northern Hemisphere Glaciation near 2.8 Ma and in the Pleistocene after 1.5 Ma. In contrast, the Hf isotope compositions (εHf) show an essentially continuous evolution from highly radiogenic values of up to +11 during the Miocene to less radiogenic present-day values (+2 to +4) during the late Quaternary. The data document a long-term transition in dominant weathering inputs, where inputs from South America are replaced by those from Southern Africa. Moreover, radiogenic peaks provide evidence for the supply of radiogenic Hf originating from Patagonian rocks to the Atlantic sector of the Southern Ocean via dust inputs.

Crustal structure of the Agulhas Ridge (South Atlantic Ocean): Formation above a hotspot?

The southern South Atlantic Ocean contains several features believed to document the traces of hotspot volcanism during the early formation of the ocean basin, namely the Agulhas Ridge and the Cape Rise seamounts located in the southeast Atlantic between 36°S and 50°S. The Agulhas Ridge parallels the Agulhas-Falkland Fracture Zone, one of the major transform zones of the world. The morphology of the ridge changes dramatically from two parallel segments in the southwest, to the broad plateau-like Agulhas Ridge in the northeast. Because the crustal fabric of the ridge is unknown relating its evolution to hotspots in the southeast Atlantic is an open question. During the RV Polarstern cruise ANT-XXIII-5 seismic reflection and refraction data were collected along a 370km long profile with 8 Ocean Bottom Stations to investigate its crustal fabric. The profile extends in NNE direction from the Agulhas Basin, 60km south of the Agulhas Ridge, and continues into the Cape Basin crossing the southernmost of the Cape Rise seamounts. In the Cape Basin we found a crustal thickness of 5.5–7.5km, and a velocity distribution typical for oceanic crust. The Cape Rise seamounts, however, show a higher velocity in comparison to the surrounding oceanic crust and the Agulhas Ridge. Underplated material is evident below the southernmost of the Cape Rise seamounts. It also has a 5–8% higher density compared to the Agulhas Plateau. The seismic velocities of the Agulhas Ridge are lower, the crustal thickness is approximately 14km, and age dating of dredge samples from its top provides clear evidence of rejuvenated volcanism at around 26Ma. Seismic data indicate that although the Cape Rise seamounts formed above a mantle thermal anomaly it had a limited areal extent, whereas the hotspot material that formed the Agulhas Ridge likely erupted along a fracture zone.

Tectonic dissection and displacement of parts of Shona hotspot volcano 3500 km along the Agulhas-Falkland Fracture Zone

In contrast to the long narrow volcanic chains in the Pacific Ocean, Atlantic hotspot tracks, in particular in the South Atlantic (e.g., Tristan-Gough, Discovery, Shona, and Bouvet), are irregular and, in some cases, diffuse and discontinuous. An important question is whether this irregularity results from tectonic dismemberment of the tracks or if it represents differences in the size, structure, and strength of the melting anomalies. Here we present new age and geochemical data from volcanic samples from Richardson Seamount, Agulhas Ridge along the Agulhas-Falkland Fracture Zone (AFFZ), and Meteor Rise. Six samples yielded ages of 83–72 Ma and are 10–30 m.y. younger than the underlying seafloor, indicating that they are not on-axis seamounts associated with seafloor spreading. The incompatible element and Sr-Nd-Pb-Hf isotopic compositions range from compositions similar to those of the Gough domain of the nearby Tristan-Gough hotspot track to compositions similar to samples from the Shona bathymetric and geochemical anomaly along the southern Mid-Atlantic Ridge (49°–55°S), indicating the existence of a Shona hotspot as much as 84 m.y. ago and its derivation from a source region similar to that of the Tristan-Gough hotspot. Similar morphology, ages, and geochemistry indicate that the Richardson, Meteor, and Orcadas seamounts originally formed as a single volcano that was dissected and displaced 3500 km along the AFFZ, providing a dramatic example of how plate tectonics can dismantle and disseminate a hotspot track across an ocean basin.

Contourite drifts as indicators of Cenozoic bottom water intensity in the eastern Agulhas Ridge area, South Atlantic

High-resolution multichannel seismic reflection profiles acquired in the Agulhas Ridge area (eastern sub-polar South Atlantic) were used in conjunction with multibeam bathymetry and Ocean Drilling Program Leg 177 borehole data to characterise deep water contourite formation in the area of the northeastern Agulhas Ridge and the Cape Rise seamounts. The transverse 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 Deep Water (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 ~33Ma and followed roughly today's 4900m depth contour. Northwest of the Cape Rise seamount we found a mounded drift with an oval shape, a height of ~400m and a width of ~50–60km 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.

Anticyclonic and cyclonic eddies of subtropical origin in the subantarctic zone south of Africa

Two eddies, one anticyclonic and the other cyclonic, intersected in the Subantarctic Zone south of South Africa during a hydrographic transect, are described using a large set of measurements including full depth hydrography, Acoustic Doppler Current Profiler velocities, biogeochemical tracers, air‐sea fluxes and altimetric sea surface height. Both eddies have a subtropical origin. The anticyclone is an Agulhas ring with convected core water of ∼12°C, and swirl velocities of 1 m s−1. It was 9.5 months old when sampled and had crossed the Agulhas Ridge. Though sampled in summer, it was releasing ∼200 W m−2 (sensible plus latent heat flux) to the atmosphere. It was observed adjacent to the Subantarctic Front, illustrating the usual encounters of such structures with this front. The cyclone, marked by pronounced low oxygen and CFC anomalies revealing an origin at the continental slope, was 4.5 months old. It had swirl speeds of 0.3 m s−1, and was coupled with the anticyclone when observed. From their kinematics and water mass properties both structures were found to transport subtropical water down to ∼900 m, the water trapped below this depth being either from the northern Subantarctic Zone, or local water. The two structures illustrate the capacity of eddies in the region to transfer subtropical and alongslope water properties into the Subantarctic Zone.

Labile Fe(II) concentrations in the Atlantic sector of the Southern Ocean along a transect from the subtropical domain to the Weddell Sea Gyre

Abstract. Labile Fe(II) distributions were investigated in the Sub-Tropical South Atlantic and the Southern Ocean during the BONUS-GoodHope cruise from 34 to 57° S (February–March 2008). Concentrations ranged from below the detection limit (0.009 nM) to values as high as 0.125 nM. In the surface mixed layer, labile Fe(II) concentrations were always higher than the detection limit, with values higher than 0.060 nM south of 47° S, representing between 39 % and 63 % of dissolved Fe (DFe). Apparent biological production of Fe(II) was evidenced. At intermediate depth, local maxima were observed, with the highest values in the Sub-Tropical domain at around 200 m, and represented more than 70 % of DFe. Remineralization processes were likely responsible for those sub-surface maxima. Below 1500 m, concentrations were close to or below the detection limit, except at two stations (at the vicinity of the Agulhas ridge and in the north of the Weddell Sea Gyre) where values remained as high as ~0.030–0.050 nM. Hydrothermal or sediment inputs may provide Fe(II) to these deep waters. Fe(II) half life times (t1/2) at 4°C were measured in the upper and deep waters and ranged from 2.9 to 11.3 min, and from 10.0 to 72.3 min, respectively. Measured values compared quite well in the upper waters with theoretical values from two published models, but not in the deep waters. This may be due to the lack of knowledge for some parameters in the models and/or to organic complexation of Fe(II) that impact its oxidation rates. This study helped to considerably increase the Fe(II) data set in the Ocean and to better understand the Fe redox cycle.

Observations of mesoscale eddies in the South Atlantic Cape Basin: Baroclinic and deep barotropic eddy variability

Anticyclones and cyclones in the eastern South Atlantic are characterized based on data collected during January 2003 to March 2005, along a Jason 1 satellite altimeter ground track, as part of the Agulhas South Atlantic Thermohaline Transport Experiment. Large and small cyclones and anticyclones were ubiquitous in the deep ocean of the eastern South Atlantic, as well as in the upper ocean. Eddy structures jointly corotating in the upper and deep water column were common; most of the time (94%) these were not axially aligned as they copropagated. The Agulhas rings and cyclones that populate the region generally carry both a steric component (baroclinic) and a mass loading component (deep barotropic structure). Average translation speeds were 7.5 cm s−1 for baroclinic eddies and twice as fast for barotropic eddies, irrespective of polarity. Translation speeds were higher than advection by the mean background flow field. In addition, large mixed baroclinic‐barotropic rings crashed into the Agulhas Ridge and nearby seamounts and split into two or more parts. Some ring parts were also observed to fuse together. Deep cyclones, as well as interactions with topography, were observed to play a role in the fission process of Agulhas rings. These processes can increase the population of Agulhas rings and their remnant eddies, which took three pathways from the Agulhas and into the Cape Basin: (1) a deep pathway between the continental slope and Erica Seamount, (2) a shallower pathway over or near the Agulhas Ridge and Schmitt‐Otto Seamount, and (3) a deep seaward pathway around the Agulhas Ridge.

Shona and Discovery Aseismic Ridge Systems, South Atlantic: Trace Element Evidence for Enriched Mantle Sources

The Shona Ridge system (Shona Ridge^Meteor Rise^Agulhas Ridge^Cape Rise seamounts) and the Discovery Rise to the north have been argued to represent the surface expressions of the Shona and Discovery mantle plumes, based on enriched samples along the southern Mid-Atlantic Ridge, a single sample from Discovery Rise and tectonic reconstructions. Leg PS69 ANT XXIII/5 of the R.V. Polarstern dredged the first comprehensive suite of samples from the Discovery Rise and Shona Ridge system, comprising alkali basalt through trachybasalt to trachyandesite and trachyte. Although many samples are moderately to heavily altered [loss on ignition (LOI)1⁄4 3^10%)], some incompatible elements (Nb, Ba, Zr, Y, Hf) appear to have remained sufficiently immobile that their ratios reflect the mantle source composition. Immobile element ratios for least altered samples (LOI52%) demonstrate the geochemically enriched nature (La/Smn 42; La/Ybn 43; Zr/Nb 510; Y/Nb 52) of the underlying mantle. Samples from Discovery Rise include primitive basalt, trachyandesite and trachyte. Mafic lavas are relatively unaltered (LOI52%) and have high La/Smn (44·5), Ba/ Nb (415) and Ba/Th (4130) coupled with low Zr/Nb (56·5) and Ce/Nbn (51). In accord with data from the single previously published Discovery Rise sample, these new samples confirm that the mantle source of Discovery Rise magmatism appears remarkably similar to that giving rise to Gough Island lavas some 400 km to the north. All lavas dredged from the aseismic Shona Ridge system are geochemically enriched relative to mid-ocean ridge basalt, with least altered samples having La/Smn42, Zr/Nb510 and Y/Nb 52. They are heterogeneous in their immobile incompatible trace element ratios, and can be subdivided into four geochemical groups: Group A and B lavas (found on the Shona Ridge and Agulhas Ridge) have Ba/Nb510 coupled with Ce/Nbn51; Group C lavas (restricted to the Meteor Rise) have Ba/Nb410 with Ce/Nbn51; Group D lavas (Meteor Rise and Cape Rise) have Ba/Nb410 and Ce/Nbn41.These differences are interpreted to reflect two physical components in the mantle source region: (1) a plume component (Shona plume; Group A and B lavas) comprising deep-seated upwelling mantle, itself heterogeneous and composed of typical, geochemically enriched, ocean island basalt-like mantle (La/Smn41; Ba/Nb 510; Ce/Nbn 51), but including domains containing a recycled oceanic sediment component (Group C lavas); (2) a delaminated and recycled, metasomatized, subcontinental lithospheric mantle component, possibly rafted within the shallow convecting asthenospheric mantle, that has given rise to the Group D lavas. These new data are consistent with an earlier postulation that the Shona Ridge system represents the surface expression of an underlying mantle plume.

Routes of Agulhas rings in the southeastern Cape Basin

Using weekly sea surface height data, Agulhas rings from the period October 1992 to December 2006 are detected and tracked, from their formation dates and throughout the Cape Basin. While 102 of them formed at the Agulhas Current retroflection, their subsequent subdivisions and junctions led to 199 trajectories. The rings geographical probability of presence shows two maxima. One, related to numerous ring passages, lies in the submarine bight formed by the Erica seamount, the Schmitt-Ott seamount, and the northeastern tip of the Agulhas Ridge. The other one, to be ascribed to topographic blocking of the eddies, is southeast of the latter obstacle. On the basis of topographic effects three routes for Agulhas rings are distinguished, a Northern route for rings that enter the south-Atlantic northeast of the Erica seamount, a Central one for those passing westward between this seamount and the tip of the Agulhas Ridge, and a Southern one farther south. Despite its bathymetric obstacles, the central route is the dominant one, both in terms of percentage of eddy crossings at its definition segment, and in terms of conveyed volume transport. Specific behaviours of rings along each route are described, referring to observations in previous studies. Some rings from the Northern route interact with the flow regime of the South African continental slope. The southernmost trajectories of the Central route are thought to settle the location of the climatological Subtropical Front in that region. The rings of the Southern route experience important core property alteration as they transit through the subantarctic domain.

Oligocene deep water export from the North Atlantic and the development of the Antarctic Circumpolar Current examined with neodymium isotopes

Neodymium (Nd) isotopes were measured on 181 samples of fossil fish teeth recovered from Oligocene to Miocene sections at Ocean Drilling Program Site 1090 (3700 m water depth) on Agulhas Ridge in the Atlantic sector of the Southern Ocean. A long‐term decreasing trend toward less radiogenic Nd isotope compositions dominates the record. This trend is interrupted by shifts toward more radiogenic compositions near the early/late Oligocene boundary and the Oligocene/Miocene boundary. Overall, ɛNd values at Agulhas Ridge are more radiogenic than at other Atlantic locations, and are similar to those at Indian Ocean locations. The pattern of variability is remarkably similar to Nd isotope results from Walvis Ridge (South Atlantic) and Ninetyeast Ridge (Indian Ocean). In contrast, Agulhas Ridge and Maud Rise Nd isotope records do not show similar patterns over this interval. Results from this study indicate that deep water in the Atlantic flowed predominantly from north to south during the Oligocene and Miocene, and that export of Northern Component Water (NCW) to the Southern Ocean increased in the late Oligocene. There is also evidence for efficient exchange of deep waters between the Atlantic sector of the Southern Ocean and the Indian Ocean, although the direction of deep water flow is not entirely clear from these data. The shifts to more radiogenic Nd isotopic compositions most likely represent increases in the flux of Pacific waters through Drake Passage, and the timing of these events reflect development of a mature Antarctic Circumpolar Current (ACC). The relative timing of increased NCW export and ACC maturation support hypotheses that link deep water formation in the North Atlantic to the opening of Drake Passage.

Tying seismic data to geologic information from core data: an example from ODP Leg 177

The integration of seismic data with core data should provide ground-truth to a structural interpretation of seismic data. The main difficulty in such an integration effort is the correct translation of physical property measurements on cores to a form which can be used in seismostratigraphic interpretation. In the absence of down-hole well data and check-shots, required knowledge of the velocity structure at the drilling locations can be obtained directly from measurements of the physical properties of core samples. This involves upscaling of the data from physical properties of cores to the sample interval used in the seismic data. In the present study, three of the seven drill-sites of ODP (Ocean Drilling Program) Leg 177 in 1997/1998, located on the Agulhas Ridge in the south-eastern Atlantic (sites 1088–1090), were connected with eight seismic profiles. Physical properties data measured on the cores from the various holes at each site were combined to create a single continuous log and used to construct synthetic seismograms. The synthetics generally show a good agreement with real seismic data in terms of amplitude and waveform. Some reflections in these generated traces may have a time-shift due to sections with incomplete or spurious P-wave velocity measurements in the ODP datasets. The main reflectors identified in the real seismic data correspond to hiatuses or periods of reduced sedimentation rates, and correlate well with density variations. One particular hiatus, clearly observable in the real seismic data, was not unequivocally identifiable in the various types of core data, and tying core data to seismic data can confirm its existence in the core data, showing the benefit of including seismic data in an interpretation of core log data. On the other hand, core data provide a calibration tool for the geological timescale of seismic data and information about the lithology, needed in the interpretation of seismic data.

Timing and Climatic Consequences of the Opening of Drake Passage

Age estimates for the opening of Drake Passage range from 49 to 17 million years ago (Ma), complicating interpretations of the relationship between ocean circulation and global cooling. Secular variations of neodymium isotope ratios at Agulhas Ridge (Southern Ocean, Atlantic sector) suggest an influx of shallow Pacific seawater approximately 41 Ma. The timing of this connection and the subsequent deepening of the passage coincide with increased biological productivity and abrupt climate reversals. Circulation/productivity linkages are proposed as a mechanism for declining atmospheric carbon dioxide. These results also indicate that Drake Passage opened before the Tasmanian Gateway, implying the late Eocene establishment of a complete circum-Antarctic pathway.

Use of multiproxy records on the Agulhas Ridge, Southern Ocean (Ocean Drilling Project Leg 177, Site 1090) to investigate sub‐Antarctic hydrography from the Oligocene to the early Miocene

Ocean Drilling Program (ODP) Site 1090, on the Agulhas Ridge in the South Atlantic sector of the Southern Ocean, is ideally located to capture changes in Southern Ocean circulation patterns. Using samples taken from cored sediments, we construct multiproxy records of productivity (biogenic barium (Baex), opal, and CaCO3 mass accumulation rates (MARs)), nutrient and organic carbon burial (reactive phosphorus (Pr) MARs), and redox conditions (U and Mn enrichments) to investigate hydrographic conditions associated with climatic shifts from the Oligocene through the early Miocene. Orbitally induced cyclicity in U and Mn enrichments (100 kyr) suggests shifts in deepwater characteristics. However, CaCO3 dissolution coincident with low U and Mn enrichments does not indicate low‐oxygen, corrosive waters similar to modern conditions. These observations indicate that a well‐developed “modern‐type” Antarctic Circumpolar Current (ACC) did not yet exist over the period from 30 to 20 Ma, with two potential consequences: The Southern Ocean was not functioning as a silica trap, permitting a broader distribution of silica that may have facilitated organic carbon burial in the ocean in general, and the lack of a deeply mixing ACC may have facilitated organic carbon burial in the Southern Ocean. Both the relative (high opal MARs coincident with low CaCO3 MARs) and absolute (high Pr MARs) burial of organic carbon suggest a powerful mechanism for pCO2 drawdown.

Cenozoic bottom current sedimentation in the Cape basin, South Atlantic

The Cape Basin is located at a central location with respect to variouswater masses. The coldest and densest of these water masses is theLower Circumpolar Deep Water.The location and topography of the Agulhas Ridge effectively blocks itsnorthward flowing branches and deflects them into a south-westerndirection to follow the bathymetric contours of the Agulhas Ridge wheresuspended sediments accumulate in contourite drifts.Several hundred metres of sediments have accumulated since the onsetof Antarctic Bottom Water intrusion into the basins of the South Atlantic.A present day like bottom current following Agulhas Ridge contours existssince an erosional event in the Early Oligocene took place which markeda prominent change in sedimentation.Strong and varying currents in the Early Oligocene formed a sheet likedeposition at high sedimentation rates.The fluctuating types of sediments produce strong seismic reflections.It was followed by a very stable unidirectional bottom current with mainlymuddy sediments. This resulted in the formation of a more than 200 kmlong, approx. 30 km wide elongate contourite drift which shows fewinternal reflections on the seismic data. An increase of well-defined unitsis found after a period of non-deposition in the Middle Miocene.Still the build-up of the drift structure continued, but at places erosion isfound, especially in Quaternary sediments and probably in relation toglaciation cycles.

Middle Eocene to early Oligocene paleoceanography from Agulhas Ridge, Southern Ocean (Ocean Drilling Program Leg 177, Site 1090)

The Agulhas Ridge, off the tip of Africa between the Atlantic and Indian Oceans, is ideally located to capture the evolution of Paleogene‐early Neogene circulation patterns associated with global cooling. Multiproxy records of productivity (biogenic barium (Baex), opal, CaCO3 mass accumulation rates (MARs)), nutrient and organic carbon burial (reactive phosphorus (Pr) MARs), and redox state of deep waters (U enrichment) from Ocean Drilling Program (ODP) Site 1090 reflect hydrographic shifts in this region between the middle Eocene and early Oligocene (∼39–33 Ma). Several peaks in increased export productivity and burial of organic matter occurred within the late Eocene (∼36.5, ∼34, and ∼33.7 Ma), which along with surface hydrologic conditions favoring opaline organisms over calcareous organisms could have aided in the draw down of pCO2 to a threshold level that facilitated large ice sheet development on Antarctica in the earliest Oligocene. Our multiproxy approach illustrates the importance of vertical as well as spatial hydrographic reorganization in amplifying or driving climatic cooling of the middle Eocene to early Oligocene by facilitating increases in the relative or absolute burial of organic carbon.