Search

Extensive ice coverage largely prevents investigations of Antarctica’s unglaciated past. Knowledge about environmental and tectonic development before large-scale glaciation, however, is important for understanding the transition into the modern icehouse world. We report geochronological and sedimentological data from a drill core from the Amundsen Sea shelf, providing insights into tectonic and topographic conditions during the Eocene (~44 to 34 million years ago), shortly before major ice sheet buildup. Our findings reveal the Eocene as a transition period from >40 million years of relative tectonic quiescence toward reactivation of the West Antarctic Rift System, coinciding with incipient volcanism, rise of the Transantarctic Mountains, and renewed sedimentation under temperate climate conditions. The recovered sediments were deposited in a coastal-estuarine swamp environment at the outlet of a >1500-km-long transcontinental river system, draining from the rising Transantarctic Mountains into the Amundsen Sea. Much of West Antarctica hence lied above sea level, but low topographic relief combined with low elevation inhibited widespread ice sheet formation.

AbstractExtensive investigation of continental rift systems has been fundamental for advancing the understanding of extensional tectonics and modes of formation of new ocean basins. However, current rift classification schemes do not account for conjugate end members formed by Large Igneous Province crust, referring to thick mafic crust, sometimes including continental fragments. Here, we investigate the rifting of William's Ridge (Kerguelen Plateau) and Broken Ridge, components of the Kerguelen Large Igneous Province now situated in the Southeast Indian Ocean, and incorporate these end members into the deformation migration concept for rifted margins. We use multichannel seismic reflection profiles and data from scientific drill cores acquired on both conjugate margins to propose, for the first time, a combined tectono‐stratigraphic framework. We interpret seismic patterns, tectonic features, and magnetic anomaly picks to determine an across‐strike structural domain classification. This interpretation considers the rift system overall to be “magma‐poor” despite being located proximal to the Kerguelen plume but suggests that syn‐rift interaction between the Kerguelen mantle plume and the lithospheric structure of William's Ridge and Broken Ridge has controlled the along‐strike segmentation of both conjugates. We integrate seismic reflection and bathymetric data to test the hypothesis of predominantly transform motion, between the Australian and Antarctic plates, in Late Cretaceous and Paleogene time.

The Agulhas Plateau (AP), along with Maud Rise (MR) and Northeast Georgia Rise (NEGR), is part of the greater Southeast African Large Igneous Province (LIP) that is hypothesized to have been emplaced above the Bouvet hotspot during breakup of Africa and East Antarctica. Since their emplacement in the Late Cretaceous, these prominent submarine plateaus have been rifted along a triple junction and have controlled connectivity between the South Atlantic, Southern Ocean, and southern Indian Ocean basins. Igneous rocks recovered on a basement high at Site U1582 (located at ~37°S) on the northern Agulhas Plateau hold clues to the age, paleolatitude, nature of LIP basement, and its relation to the mid-ocean ridges that separated the AP from MR and NEGR. At Site U1582, a pillow basalt sequence with intercalated sediments was recovered. Based on initial shipboard biostratigraphy, a Santonian age (~85 Ma) was assigned to this unit. The moderately altered, mildly porphyritic mafic igneous basement rocks recovered at Site U1582 contain abundant veins and carbonate-filled voids. We report in situ U-Pb ages of carbonate vein and void fills (n=20) along with paleomagnetic directions (n=25) from the basaltic basement. The oldest carbonate formed at ~95 Ma (Cenomanian), which we interpret as early diagenetic ages that immediately postdate LIP emplacement during cooling and associated fracturing. Younger vein generations reflect basement carbonation due to seawater circulation, which prevailed until long after LIP emplacement. Basaltic basement rocks at Site U1582 record magnetic field directions reliably. Paleomagnetic analysis yields a negative inclination (normal polarity), which we correlate with Chron C34n (Cretaceous Normal Superchron). We calculate a ~45-50°S preliminary mean paleolatitude that allows us to refine plate kinematic reconstructions and to test the genetic relationship between the AP, MR and NEGR. Our combined age-paleolatitude dataset has implications for Cretace

Core samples recovered from the Transkei Basin (Hole U1581B), offshore South Africa, during IODP Expedition 392 include an expansive record of organic-rich sediments from the Campanian to Paleocene (~74-63 Ma). Investigation of the biomarker composition of this sequence revealed significant amounts of polycyclic aromatic hydrocarbons (PAH) reflecting terrestrial sources of organic matter (OM) coupled with evidence for wildfires. Perylene, derived from diagenetic alteration of terrestrial OM, is the dominant PAH in the Campanian sediments. Other prominent PAH components are tetrahydrochysenes and tetrahydropicenes, which are formed by early-stage diagenetic aromatization, with or without loss of the functionalized A-ring, of triterpenoid lipids. The co-occurrence of their precursor alkenes and ketones afforded further evidence for inputs of angiosperm-derived terrigenous OM, whereas the absence of diterpenoids suggests minimal contributions of terrestrial OM from gymnosperms. The dominance of OM originating from angiosperms throughout this interval suggests that these plants were well-established as the principal vegetation in southern Africa following their expansion during the Late Cretaceous. The presence of coronene in the sedimentary succession indicates that the sources of terrestrial OM also include pyrogenic material because this compound is formed during intense combustion and therefore serves as a sedimentary marker for wildfires. Coronene occurs as a minor PAH component in Campanian samples, but it is a prominent PAH component of the Paleocene samples. It is dominant in the basal Paleocene sample consistent with inputs from global wildfires at the K/Pg boundary, confirming their prevalence at high southern latitudes, whereas the lower proportion of perylene in this interval may reflect a diminished supply of unburnt lignin precursors. Coronene remains a substantive component in later Paleocene samples suggesting the protracted occurrence of wildfires. In a

The stability of the West Antarctic Ice Sheet is threatened by the incursion of warm Circumpolar Deepwater which flows southwards via cross-shelf troughs towards the coast there melting ice shelves. However, the onset of this oceanic forcing on the development and evolution of the West Antarctic Ice Sheet remains poorly understood. Seismic reflection profiles image sediment bodies in troughs on the shelf of the Amundsen Sea Embayment, which possess the geometry and depositional pattern of plastered sediment drifts. Tentative dating of one drift via a seabed drill core suggests a formation age of this sediment body to be around the Eocene-Oligocene. We suggest this indicates a southward inflow of deep water which probably supplied heat and, thus, prevented West Antarctic Ice Sheet advance. We conclude that the West Antarctic Ice Sheet has likely experienced a strong oceanic influence on its dynamics since its initial formation.

IODP Expedition 392 to the Agulhas Plateau (AP) recovered sedimentary and igneous sequences from four sites (Sites 392-U1579, 392-U1580, 392-U1581, and 392-U1582) ranging in age from the Late Cretaceous to the Pleistocene. The primary objectives of this expedition were to examine the nature of the AP basement, the opening of oceanic gateways, and the evolution of the climate system through the Cretaceous hothouse and into the Cenozoic. A key to achieving these objectives is the development of high-quality age models for the sedimentary and igneous sequences recovered from each site. Shipboard age models were developed using a combination of biostratigraphic age constraints, in addition to magnetostratigraphy. To improve upon the age model, shore-based paleomagnetic analysis of discrete samples was performed on intervals where polarity could not be confidently determined from shipboard archive half measurements, specifically focused on intervals where refined age models help achieve the Expedition objectives. Rock and environmental magnetic analysis was also performed on select discrete samples to characterize changes in magnetic mineralogy and grain size throughout the sedimentary sequence captured in each hole. Results from rock magnetic experiments help assess the reliability of measured magnetic signals and further can be used to say something about paleoenvironmental conditions. Magnetic minerals are responsive to many environmental changes including changes in sediment source, redox, weathering, and paleooceanographic conditions and can be utilized as a powerful tool for investigating past environments. Magnetic mineralogic changes will be connected to results from pore water geochemistry and astronomical tuning to help further understand the processes behind the observed changes. Here, we will present on the updated magnetostratigraphy and preliminary rock and environmental magnetic analyses.

Seafloor weathering of Agulhas Plateau Large Igneous Province volcaniclastics as a driver of ocean chemistry

Climate models have identified significant geography-related Cenozoic cooling arising from the opening of Southern Ocean gateways (e.g., Sauermilch et al., 2021). For example, a gradual strengthening of the Antarctic Circumpolar Current (ACC) has been proposed as the primary cause of cooler deep ocean temperatures associated with the transition from the Cretaceous "Supergreenhouse" to the Oligocene icehouse (e.g., Sijp et al., 2014). IODP Expedition 392 'Agulhas Plateau Cretaceous Climate' drilled four sites in the African-Southern Ocean gateway in 2022 to significantly advance understanding of how temperatures, ocean circulation, and sedimentation patterns evolved as CO2 levels rose and fell and Gondwana broke up (Uenzelmann-Neben et al., 2022). Prior to Exp 392, seismostratigraphic models developed for the African-Southern Ocean gateway relied solely on age data from remote drilling sites and information from piston cores, gravity cores, and dredge samples. As a result, a high level of uncertainty had to be assumed for reflector age information, particularly for the sparsely sampled pre-Miocene sedimentary column. We here present preliminary findings from a revised seismostratigraphy based on core-log-seismic integration at Sites U1579 (central Agulhas Plateau) and U1580 (southern Agulhas Plateau), both of which are located at the heart of the African-Southern Ocean gateway. Core density and velocity data were edited, corrected to in-situ conditions, and combined with downhole logging data. Synthetic seismograms provide an accurate traveltime to depth conversion and show an excellent correlation of drilling results with the site survey seismic field records. A first correlation of the seismic reflection data with the preliminary shipboard bio- and magnetostratigraphy reveals that the published seismostratigraphic models are erroneous and need revisions, for example, a reflector interpreted to represent a lower Eocene sea level highstand on the Agulhas Plateau has

Campanian to Paleocene organic-rich sediments (~74-60 Ma) recovered from the Transkei Basin (Hole U1581B), offshore South Africa, during IODP Expedition 392 contain suites of C37-C40 alkenones derived from haptophyte algae that extend the temporal continuity of their occurrences and expand their paleogeographic range to high southern latitudes (~58°S) during this time interval. Alkenone profiles are broadly similar throughout the stratigraphic section with the similarity between Maastrichtian and Danian samples indicating a conformity in biosynthetic pathways across the K/Pg boundary. Thus, the source haptophytes for alkenones survived and subsequently recovered after the extinction event, consistent with temporal trends for assemblages of calcareous nannoplankton from the southern hemisphere. The lineages of specific alkenones record evolutionary developments in their biosynthetic pathways. The occurrence of a methyl C39:2 alkenone in the Paleocene and both methyl and ethyl C38 and C39 alkenones in the Campanian extends the range of occurrence of alkenone with carbonyl groups at multiple positions, and the required duality in their biosynthetic pathways. The dominance of the C40 alkadien-3-one in several samples contrasts with its scarcity in Neogene marine sediments and presence among extant haptophytes. C40 alkenones are prevalent constituents of coastal and lacustrine species in phylogenic Group II, notably Isochrysis, but have only once been reported in marine species from phylogenic Group III. The sporadic prominence of C40 alkenones prior to the early Eocene seems to reflect a broader suite of active biosynthetic pathways than those expressed by extant marine haptophytes. Thus, Cretaceous through Paleocene marine sediments may reflect alkenone contributions from both Isochrysidaceae (Group II) and Noelaerhabdaceae (Group III) following their divergence in the Early Cretaceous. The accompanying C40:3 alkenone contrasts with the absence of other alkatrienones p

Climate models have identified significant geography-related Cenozoic cooling arising from the opening of Southern Ocean gateways (e.g., Sauermilch et al., 2021). For example, a gradual strengthening of the Antarctic Circumpolar Current (ACC) has been proposed as the primary cause of cooler deep ocean temperatures associated with the transition from the Cretaceous "Supergreenhouse" to the Oligocene icehouse (e.g., Sijp et al., 2014). IODP Expedition 392 'Agulhas Plateau Cretaceous Climate' drilled four sites in the African-Southern Ocean gateway in 2022 to significantly advance understanding of how temperatures, ocean circulation, and sedimentation patterns evolved as CO2 levels rose and fell and Gondwana broke up (Uenzelmann-Neben et al., 2022). Prior to Exp. 392, seismostratigraphic models developed for the African-Southern Ocean gateway relied solely on age data from remote drilling sites and information from piston cores, gravity cores, and dredge samples. As a result, a high level of uncertainty had to be assumed for reflector age information, particularly for the sparsely sampled pre-Miocene sedimentary column. We here present preliminary findings from a revised seismostratigraphy based on core-log-seismic integration at Sites U1579 (central Agulhas Plateau) and U1580 (southern Agulhas Plateau), both of which are located at the heart of the African-Southern Ocean gateway. Core density and velocity data were edited, corrected to in-situ conditions, and combined with downhole logging data. Synthetic seismograms provide an accurate traveltime to depth conversion and show an excellent correlation of drilling results with the site survey seismic field records. A first correlation of the seismic reflection data with the preliminary shipboard bio- and magnetostratigraphy reveals that the published seismostratigraphic models are erroneous and need revisions, for example, a reflector interpreted to represent a lower Eocene sea level highstand on the Agulhas Plateau has

The Kerguelen Plateau, southern Indian Ocean, which rises up 2000 m above the surrounding seafloor, forms an obstacle for the flow of the Antarctic Circumpolar Current (ACC) and Antarctic Bottomwater (AABW). The ACC is strongly deviated in its flow towards the north. A branch of the AABW flows northwards along the eastern flank of the plateau and in its path is steered by several basement highs and William’s Ridge. Seismic data collected during RV Sonne cruise SO272 image sediment drifts shaped in the Labuan Basin, which document onset and variabilities in pathway and intensity of this AABW branch in relation to the development of the Antarctic ice sheet and tectonic processes, e.g., the opening of the Tasman gateway. The eastern flank of the Kerguelen further shows strong erosion of the post-mid Eocene sequences. In places, the Paleocene/early Eocene sequences are also affected by thinning and erosion. A moat can be observed along the Kerguelen Plateau flank indicating the flow path of the north setting AAWB branch. Sediment drifts and sediment waves are formed east of the moat. Similar features are observed at the inner, western flank of William’s Ridge thus outlining the recirculation of the AABW branch in the Labuan Basin. The chronological and spatial will be reconstructed via the analysis of those sedimentary structures to provide constraints on climate and ocean circulation variability.

3300 km of newly collected 2D seismic reflection data across the Falkland Plateau, acquired during cruise MSM81 (2019) allowed for mapping of two distinct bottom-simulating-reflection (BSR) features, which are mostly pronounced within the western and southern sectors of the Falkland Plateau Basin (FPB-BSR) and the eastern sector of the Falkland Trough (FT-BSR). The nature of these BSRs is investigated by means of their reflection characteristics and seismic expressions and is concluded to be associated with silica diagenetic fronts. In absence of a proximal borehole, age information is derived through correlation with the Deep Sea Drilling Project Leg 36 Sites 327 and 330 and Leg 71 Site 511, on the easternmost proximity of the FPB, along the seismic profiles. It is argued in this study, that despite their similar origin, presumably in connection to an Opal A/CT diagenetic front, FPB- and FT-BSR display dissimilar geometrical characters. While the FPB-BSR is by definition a true BSR, which mimics the present seafloor, the geometrical extent of the FT-BSR, which shows parallelism with a shallower buried (Early/Middle Miocene?) reflector, favors a fossilized diagenetic front parallel to a paleo-isotherm. Palaeoceanic and palaeomorphologic implications are brought based on observed depth properties of the BSRs by considering different scenarios for the geothermal gradients. The observations suggest the absence of up to a few hundred meters of sedimentary deposits which presumably have been eroded due to the erosive action of the bottom currents.

The Paleocene-Eocene Thermal Maximum (PETM) was an abrupt and extreme warming event associated with rapid input of light carbon into the ocean-atmosphere system. The carbon cycle perturbations during the PETM caused significant changes to marine plankton, including extinction of some benthic foraminifers and the appearance of malformed calcareous nannoplankton, possibly related to ocean acidification during the event. The PETM is now considered a potential analog for the effects of anthropogenic climate change due to its rapid onset; thus, study of PETM records offers an opportunity to better understand the potential effects of rapid climate change on marine phytoplankton communities. Here we present calcareous nannofossil assemblage data across a newly recovered PETM section from International Ocean Discovery Program (IODP) Site U1580, drilled on the central Agulhas Plateau during IODP Expedition 392 in early 2022. Present water depth at this site is 2560 m and it sits at 40° 47.15’S, although the site has moved progressively northward since the Agulhas Plateau formed in the mid-Cretaceous, when the site was located about 20° further south. The PETM interval was identified during the expedition by a change in sediment color, increase in magnetic susceptibility, and calcareous nannofossil assemblages. Low-resolution bulk 13C measurements conducted following the expedition confirm a negative isotope excursion and decrease in calcium carbonate content across the interval. The shipboard age model based on nannofossil and planktonic foraminifer biostratigraphy, together with magnetostratigraphy, suggests relatively high sedimentation rates (~2.5 cm/kyr). Nannofossil assemblages are quite well preserved, with PETM-specific taxa present including Rhomboaster calcitrapa, Rhomboaster cuspis, Rhomboaster bramlettei, Discoaster araneus, and Discoaster acutus. Initial qualitative analyses indicate Zygrhablithus bijugatus is particularly abundant within the PETM interval, wher

International Ocean Discovery Program (IODP) Expedition 392 cored three sites on the Agulhas Plateau and one site in the Transkei Basin to address questions regarding the origin and timing of emplacement of Agulhas Plateau, as well as examine Southern Ocean climate history and opening of oceanic gateways from the Cretaceous through the Paleogene. Age models for the sites rely primarily on calcareous nannofossils and magnetostratigraphy, with dinoflagellates providing key events for some intervals, and additional contributions from planktonic foraminifers and diatoms. Site U1579, located in a basin on southern the central Agulhas Plateau, records a nearly continuous section dated to the Santonian to earliest Miocene. Dinoflagellates provide age control for the zeolitic sandstone and siltstone with glauconite at the base of the cored section. Above this, nannofossils are common to abundant and moderately preserved in upper Santonian to Maastrichtian calcareous chalks and the assemblages show Southern Ocean affinities. Paleogene nannofossils are abundant and moderately to well preserved. Sedimentation rates were lowest in the Eocene, which includes either condensed intervals or hiatuses. Nannofossils are well preserved in the Oligocene and assemblages consist of primarily mid-latitude species with occasional incursions of cold-water taxa. Site U1580 is also located on the southern Agulhas Plateau adjacent to a basement high. This site records several unconformities, and the lowermost part of the cored interval is interspersed with basalt layers interpreted as sills. The oldest sediment is likely uppermost Cenomanian in age. Overlying the shallowest basalt is Coniacian–Santonian silt- and sandstone with varying proportions of zeolites, glauconite, and carbonate. Sedimentation rates were very high (~10 cm/ky) during this time. Much of the early Campanian and mid-Maastrichtian is missing at this site. Paleocene nannofossils are moderately well preserved and suggest a cont

This presentation gives an account of the International Ocean Discovery Program (IODP) expedition 392 [Agulhas Plateau Cretaceous Climate: drilling the Agulhas Plateau and Transkei Basin to reconstruct the Cretaceous–Paleogene tectonic and climatic evolution of the Southern Ocean basin] which took place from February to April 2022. Four geological sites were drilled on the seafloor of the Agulhas Plateau and the Transkei Basin (Figure 1). A total of 28 scientists from sailed on the Joides Resolution vessel and the deepest hole drilled of four reached 994 m below the seafloor, in over 4500 m of water. The overarching objectives of Expedition 392’s recovery of rocks and sedimentary sequences were aimed at: (1) determining the nature and origin of the Agulhas Plateau; (2) advancing the understanding of how Cretaceous temperatures, ocean circulation, and sedimentation patterns evolved as CO2 levels rose and fell and the breakup of Gondwana progressed; (3) documenting long-term palaeoceanographic variability through the Late Cretaceous and Paleogene; and (4) investigating geochemical interactions between igneous rocks, sediments, and pore waters through the life cycle of a large igneous province.

A series of organic-rich sedimentary sequences that span the Paleocene through the Campanian was recovered from the Transkei Basin during IODP Expedition 392. Preliminary analyses of their organic geochemistry reveals suites of biomarkers that reflect diverse contributions of organic matter from both marine and terrestrial sources. The characteristics of the organic matter (OM) for a range of samples have been determined by shipboard and shore-based analyses that include evaluation of OM type and maturity by Rock-Eval pyrolysis, isotopic compositions of OM and the distributions of biomarkers including GDGTs. Samples from both the Paleocene and Campanian contain series of C37 to C40 diunsaturated alkenones, including the earliest record of C38 alkadien-2-ones and C39 alkadien-3-ones [1]. A sequence of Campanian samples also reveal an abundance of C28 sterenes and steryl ethers potentially indicative of upwelling conditions [2]. Polyaromatic hydrocarbons, including coronene, are present throughout the sequence likely reflecting inputs of OM associated with paleofires [3]. Variations in the abundance of organosulfur compounds [4,5] may afford evidence for temporal changes in redox conditions within the Transkei Basin during the Campanian. These initial results provide evidence that biomarker profiling will facilitate determination of the sources of OM throughout Late Cretaceous and Paleocene and the paleoenvironmental conditions of its deposition. [1] Brassell, 2014; [2] Brassell, 2009; [3] Finkelstein et al., 2005; [4] Valisolalao et al., 1984; [5] Brassell et al., 1986

3.9.2. haiku captures scientific exploration and discovery as it took place in remote locales, far from the public eye. It invites audiences of all ages to engage with cores from the privileged point-of-view of researchers who were the first to see, smell and touch material from deep under the ocean floor; who, after watching the cores be split open, spent hours analyzing each centimeter. Their poems, and especially their readings of them, reveal not only the cores but also the scientists as they experience anticipation, frustration, awe, excitement and wonder. It turns out that haiku is an especially relevant way to package stories from cores of mud and rock from the Cretaceous period, which was the focus of IODP Expedition 392 to the Agulhas Plateau. An ancient poetic form consisting of 17 syllables in three lines (5-7-5), haiku compactly and powerfully captures moments of time . . . moments that, on the surface, may appear unremarkable—a frog jumping. Moments that resonate with the deep truths that emerge from paying close attention to the natural world. When introduced to the concept of 3.9.2 haiku, more than one brilliant researcher balked: “I’m no good at this. . . I can’t write poetry.” During several writing workshops conducted during the expedition, scientists began to recognize haiku as the linguistic equivalent to cores they were studying. They delighted that a dozen (or fewer) precisely packaged words could align so closely with the cores precisely drilled from the Agulhas Plateau. Each individual haiku is a gem. Strung together, they are a trove of impressions that span millennia.