ALBERT

Description: Listed are edited, spliced, and in situ corrected data vs. various depth scales, the shipboard age scale and two-way-traveltime (TWT). The shipboard age model for Site U1475 was derived from time estimates based on a combination of major planktonic foraminifer, calcareous nanno-plankton, diatom, and paleomagnetic datums. K (wt.%) is derived from measurements of natural gamma radiation (NGR) according to De Vleeschouwer, D., Dunlea, A. G., Auer, G., Anderson, C. H., Brumsack, H., de Loach, A., et al. (2017). Quantifying K, U, and Th contents of marine sediments using shipboard natural gamma radiation spectra measured on DVJOIDESResolution. Geochemistry, Geophysics, Geosystems, 18(3), 1053-1064, doi:10.1002/2016GC006715. Description of the depth scales: The CSF-A scale (here DEPTH, sediment/rock) is specific to each hole of a site and combines the drilling depth below seafloor of the core top depth and the curated depth within a core after retrieval. The CSF-A depth scale is equivalent to the meters below seafloor (mbsf) scale known from, e.g., the Ocean Drilling Program (ODP). For the construction of the most representative single continuous sedimentary section, intervals from multiple holes need to be spliced together. This requires a common, composite depth scale (CCSF, formerly known as mcd) for a given IODP Site which is based on the correlation of coeval, laterally continuous features seen in the physical properties in all drilled holes (which will generally occur at different depths on the CSF-A depth scales for each hole). Once such correlative features (tie-points) were identified at Site U1475 the depth of individual cores was offset relative to CSF-A in that hole, resulting in the Composite Curated depth below Sea Floor (CCSF-A) depth scale (here Depth composite) for each hole. By combining selected intervals from Holes U1475B, C, E and F between the previously established tie points a complete stratigraphic section (splice) was constructed. The designated depth scale of the splice is the CCSF-D scale. CCSF-D applies only to intervals included in the splice and intervals not included in the primary splice retain the CCSF-A scale. The CCSF-A scale does not result in alignment of all coeval features because of the differing effects of coring-induced stretching and squeezing among cores, as well as sedimentological differences between holes. For our study it was desirable to map into the splice those intervals not included in the splice itself. We accomplished this by simultaneously comparing color reflectance (b*) and NGR for all holes with subsequent identification of correlative tie points at the highest possible resolution and linear adjustments of depths between ties. This adjusted depth is designated as the CCSF-C depth scale (here Depth composite revised), and to the extent that the correlations are accurate, the CCSF-C and CCSF-D scales are equivalent. CCSF-C and -D typically exceed the in situ depth (core depth below sea floor CSF-A) by ~10%. The growth rate of the CCSF-D depth scale, relative to CSF-A is on average 9.5% for all holes. Thus to correct for the depth offset a linear compression - Depth (m CCSF-B) = Depth (m CCSF-C)/1.095 - was applied to the entire depth so that the compressed core length was equal to the interval cored. The CCSF-B scale (here Depth composite corrected) hence indicates the in situ depth in meters below the seafloor (mbsf).

Description: In 2016 the International Ocean Discovery Program (IODP) Expedition 361 (“SAFARI”) recovered complete high-resolution Plio-/Pleistocene sediment sections at six drilling locations on the southeast African margin and at the oceanic connection between the Indian and South Atlantic Oceans. Site U1475 is located on the southern flank of the Agulhas Plateau, proximal to the entrance of North Atlantic Deep Water (NADW) to the Southern Ocean and South Indian Ocean. The site was drilled into a sediment drift in 2669 m water depth and comprises a complete carbonate rich (74 – 85%) stratigraphic section of the last ~7 Ma. The contourite deposits hold detailed information on past changes in the bottom water flow history in the Indian-Atlantic ocean gateway. Here we present results from the integration of physical properties, seismic reflection data, and major element records. The whole spliced sediment record (292 meters) of Site U1475 was measured using an X-ray fluorescence (XRF) core scanner to derive multi-centennial resolution records of major element intensities. Based on these measurements it is possible to derive biogenic (e.g. %CaCO3) and siliciclastic (e.g. TiO2, K2O) mineral phases. Elemental log-ratios, such as Ca/Ti and K/Fe, reflect variations in biogenic (CaCO3) vs. terrigenous supply and variability of the terrigenous provenance, respectively. While long-term changes in physical properties and elemental ratios can be linked to the seismic reflection patterns associated with deep water circulation changes, short-term cyclicities reflect Plio-Pleistocene climate variations at Milanlovitch-frequencies. Evolutionary spectra show that the orbital control on sediment composition was variable over time. During the last 4 Ma energy is concentrated at the 41ka band of obliquity and at lower frequencies. In contrast, the orbital precession cycle (19-23ka) is very prominent in a peculiar high sedimentation rate interval in the early Pliocene (~4 to 5 Ma) that is bounded by seismic reflectors and characterized by the development of sediment waves.

Description: An important element of the global ocean thermohaline circulation is the oceanic connection between the Indian and South Atlantic Oceans off South Africa. Variable amounts of warm, salt-enriched South Indian Ocean waters enter the South Atlantic, the so-called ‘warm water return route’, and provide a source for heat and salt to the Atlantic thermocline that ultimately preconditions the Atlantic meridional overturning circulation for convection in the north, the formation of North Atlantic Deep Water (NADW). This eastward surface return flow is compensated at depth by a westward setting deep flow into the southern Indian Ocean that consists of NADW exiting the South Atlantic and Southern Source Waters (SSW), influenced by the Antarctic Circumpolar Current (ACC). Here we present a high-resolution multi-proxy record of deep water variability from sediment core MD02-2588 (2907 m water depth) and IODP Site U1475 (2669 m water depth) both recovered from the southern flank of the Agulhas Plateau in the southernmost South Atlantic. The location is close to the interface between NADW and SSW in the Southern Ocean enabling the reconstruction of the timing and amplitude of changes in southward advection of NADW and Southern Ocean circulation. We concentrate on identifying the phasing between changes in ice volume, the location of surface ocean fronts, deep ventilation and near-bottom flow speeds over the past 1.5 Ma – across the Middle Pleistocene transition. Our benthic carbon isotope record from MD02-2588/Site U1475 strongly suggest that there was a continued mid-depth northern source water influence over the southern Agulhas Plateau during glacial periods of the past 1.5 Ma. Nonetheless, significantly increased near bottom flow speeds, ~5–10 cm s−1 (3–7 μm coarser), during glacial periods indicates that there must be additional controls on physical ventilation. We suggest that vigor of near bottom currents on the Southern Agulhas Plateau is likely influenced by the orbital scale meridional expansion and contraction of the ACC and its associated surface fronts.

Description: Contourite deposits found in the Indian-Atlantic ocean gateway hold detailed information on past changes in the bottom water flow history over long time intervals of the Cenozoic. Until IODP Exp. 361 only late Pleistocene paleoceanographic studies for the region were carried out using sediment samples obtained from piston cores. We present preliminary results from Site U1475 (Agulhas Plateau), a location proximal to the entrance of North Atlantic Deep Water (NADW) to the Southern Ocean and South Indian Ocean. The site is located over a sediment drift in 2669 m water depth and comprises a complete stratigraphic section of the last ∼7 Ma. The whole spliced sediment record (292 meters) of Site U1475 was measured using an X-ray fluorescence (XRF) core scanner to derive multi-centennial resolution records of major element intensities. Based on these measurements it is possible to derive of biogenic (e.g. %CaCO3) and siliciclastic (e.g. TiO2, K2O) mineral phases. Because the Ca counts almost exclusively stem from biogenic carbonate formed by microfossil shells the ratio of biogenic components vs. terrigenous is reflected in e.g. the Ca/Ti or Ca/Fe records. On the other hand, changes of elemental ratios such as K/Fe or Al/Ti show the variability within the terrigenous sediment fraction. While long-term changes in elemental ratios can be linked to the seismic reflection pattern associated with deep water circulation changes, short-term cyclicities in sediment provenance reflect orbital scale Plio-Pleistocene climate variations. E. g. power spectra performed on the ln(Ca/Ti) and ln(K/Fe) records for a peculiar Pliocene (~5.7 - 4.1 Ma) high sedimentation rate interval reveal significant spectral density peaks at periods close to the Milankovitch precession band (19 – 23 kyr). Such a high variability in the precession band is also evident in a number of other element ratios while changes in physical properties (e.g. density, seismic impedance) for the same interval seem to be dominated by eccentricity. We present evolutionary spectral analyses revealing how the orbital response of the different parameters have changed over time and derive an improved age model based on cyclostratigraphy.

Description: The gateway south of South Africa constitutes an integral inter-ocean link in the global thermohaline circulation (THC) since it allows the exchange of shallow- and deepwater masses between the Indian and the Atlantic. Thus understanding past variations of this current system is important for improving our knowledge of the global climate. The long-term changes in deepwater flow in the Atlantic-Indian gateway during the Cenozoic have been initially studied using reflection seismic profiles. But in many cases the seismic stratigraphy is poorly constrained and not further resolved within the time period from the late Miocene to present. In particular, there are limited Pliocene records that can be used to investigate the influence of climatic (e.g. Antartic ice volume) and tectonic (e.g. closure of the central American seaway) on the deep-water variability. Here we focus on the bottom water flow around the Agulhas Plateau, a location proximal to the entrance of North Atlantic Deep Water (NADW) to the Southern Ocean and South Indian Ocean. IODP Expedition 361 (SAFARI) Site U1475 was drilled in 2669 m water depth into a sediment drift that is deposited on the southwestern flank of Agulhas Plateau and comprises a complete stratigraphic section of the last ∼7 Ma. We present cleaned, edited, and spliced high-resolution data sets of sediment physical properties measured at Site U1475. Synthetic seismograms generated from the velocity and bulk density core scanning records allow a detailed correlation oft the drilling results with the Site survey seismic reflection profiles. Seismic reflectors at 3.75 and 3.87 s (two-way-traveltime) correspond to major increases in acoustic impedance at ∼110 and ∼216 meters below seafloor. Based on the preliminary shipboard biostratigraphic age model sediments at these depths have ages of ∼4.0 and ∼5.1 Ma, respectively. Furthermore spectral analyses of physical property records such as natural gamma radiation and colour reflectance reveal climate variability on orbital and suborbital timescales.

Description: 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 today; 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) shoaled. 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 two thousand 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.

Description: The teleconnections existing between low- and high-latitude water masses are a critical component of the Earth’s climate system. One region of global significance is the Indian-Atlantic Ocean gateway (I-AOG) that lies off the southern tip of Africa. Today, the regional oceanography is dominated by the Agulhas Current and its leakage into the South Atlantic, the so-called “warm water route,” the Agulhas Current Retroflection and the Agulhas Return Current’s interaction with the subtropical front (STF). However, the connections between these frontal systems and links to high latitude climate dynamics are not well understood. On glacial/interglacial timescales, biogenic silica (BSi) production in the I-AOG, close to the STF, has been linked to transport of high-latitude silica rich intermediate waters to this site throughout the Pleistocene (Romero et al., 2015). Here we present BSi accumulation rates, diatom assemblage data, and bulk sediment chemistry (XRF) for Pliocene age sediments from IODP Site U1475 (41°25.61’S; 25°15.64’E, 2669 m water depth). While the overall BSi content remains low (0.5 to 4.18 weight %), increases in diatom accumulation are seen at four distinct intervals (4.6 to 4.3 Ma, 3.3 Ma, 3.14 Ma, and 2.8 to 2.6 Ma). Additionally, XRF records of Si/Al and Ba/Al appear to coincide with the BSi record and we infer that nutrient-rich intermediate waters “leaked” from the Southern Ocean and resulted in increased BSi production. Additionally, an increase in opal accumulation at the Site between 4.6 to 4.3 Ma, is likely tied to a reorganization of global nutrient pools associated with the shoaling of waters at the Panama Canal. During the shoaling event BSi production increases at Site U1475 are coincident with a reduction in BSi production in the Galapagos Basin (e.g. ODP Site 846), thus suggesting a potential connection between Pacific and Atlantic Ocean basins during the Pliocene. An increase in abundance of Southern Ocean diatoms from 3.9 Ma to 2.6 Ma may also indicate water mass cooling and increased transport of high-latitude intermediate waters. The Site U1475 record of microfossils highlights a trend of intermediate water input throughout the Pliocene. Additionally, comparisons with the so-called “cold water route” will allow a better view of global intermediate water changes during the Pliocene.