Description: Paleobathymetric models and sediment thickness estimations give insight into the development of the Antarctic Ice Sheets, Southern Ocean current systems and opening of the Southern Ocean Gateways since the Eocene/Oligocene Boundary. A more sophisticated assessment of the paleobathymetry and the sediment deposition in the Southern Ocean are essential for robust reconstructions of paleoclimate and paleooceanographic scenarios. For this reconstruction, we used all available reflection seismic data (refer to Hochmuth et al. (2020) Fig. 1 for data coverage) and borehole information within the Southern Ocean. Paleobathymetric grids are calculated using the backstripping method as implemented in the BalPal code (Wold et al. 1992). Plate tectonic reconstruction follows Müller et al. (2018). The following paleobathymetric reconstruction time slices are available: Eocene/Oligocene Boundary (34 Ma), early Oligocene (27 Ma), Oligocene/Miocene Transition (24 Ma), early Miocene (21 Ma), middle Miocene (14 Ma), late Miocene (10.5 Ma), early Pliocene (5 Ma), Pliocene/Pleistocene Boundary (2.6 Ma) and middle Pleistocene (0.65 Ma). The sediment thickness maps presented here include pre- and post-34 Ma sediment deposition as well as the specific time intervals of the early Oligocene (34 Ma – 27 Ma), late Oligocene to early Miocene (27 Ma – 24 Ma), early Miocene (24 Ma – 21 Ma), early to middle Miocene (21 Ma – 14 Ma), middle to late Miocene (14 Ma – 10.5 Ma), late Miocene (10.5 Ma – 5 Ma), Pliocene (5 Ma – 2.65 Ma) and Pleistocene (2.6 Ma to recent).

Description: Here we present the first radiometric age data and a comprehensive geochemical data set (including major and trace element and Sr-Nd-Pb-Hf isotope ratios) for samples from the Hikurangi Plateau basement and seamounts on and adjacent to the plateau obtained during the R/V Sonne 168 cruise, in addition to age and geochemical data from DSDP Site 317 on the Manihiki Plateau. The 40Ar/39Ar age and geochemical data show that the Hikurangi basement lavas (118-96 Ma) have surprisingly similar major and trace element and isotopic characteristics to the Ontong Java Plateau lavas (ca. 120 and 90 Ma), primarily the Kwaimbaita-type composition, whereas the Manihiki DSDP Site 317 lavas (117 Ma) have similar compositions to the Singgalo lavas on the Ontong Java Plateau. Alkalic, incompatible-element-enriched seamount lavas (99-87 Ma and 67 Ma) on the Hikurangi Plateau and adjacent to it (Kiore Seamount), however, were derived from a distinct high time-integrated U/Pb (HIMU)-type mantle source. The seamount lavas are similar in composition to similar-aged alkalic volcanism on New Zealand, indicating a second wide-spread event from a distinct source beginning ca. 20 Ma after the plateau-forming event. Tholeiitic lavas from two Osbourn seamounts on the abyssal plain adjacent to the northeast Hikurangi Plateau margin have extremely depleted incompatible element compositions, but incompatible element characteristics similar to the Hikurangi and Ontong Java Plateau lavas and enriched isotopic compositions intermediate between normal mid-ocean-ridge basalt (N-MORB) and the plateau basement. These younger (~52 Ma) seamounts may have formed through remelting of mafic cumulate rocks associated with the plateau formation. The similarity in age and geochemistry of the Hikurangi, Ontong Java and Manihiki Plateaus suggest derivation from a common mantle source. We propose that the Greater Ontong Java Event, during which ?1% of the Earth's surface was covered with volcanism, resulted from a thermo-chemical superplume/dome that stalled at the transition zone, similar to but larger than the structure imaged presently beneath the South Pacific superswell. The later alkalic volcanism on the Hikurangi Plateau and the Zealandia micro-continent may have been part of a second large-scale volcanic event that may have also triggered the final breakup stage of Gondwana, which resulted in the separation of Zealandia fragments from West Antarctica.

Description: 2 new deglacial planktic boron isotope records from the Southwest Pacific. pH/pCO2 calculated from 12 boron isotope records from the global ocean were compiled to determine the history of air-sea exchange of CO2 since the glacial (last 25ka).

Description: Seafloor pockmarks of varying size occur over an area of 50,000 km² on the Chatham Rise, Canterbury Shelf and Inner Bounty Trough, New Zealand. The pockmarks are concentrated above the flat-subducted Hikurangi Plateau. Echosounder data identifies recurrent episodes of pockmark formation at ~100,000yr frequency coinciding with Pleistocene glacial terminations. Here we show that there are structural conduits beneath the larger pockmarks through which fluids flowed upward toward the seafloor. Large negative Δ¹⁴C excursions are documented in marine sediments deposited next to these subseafloor conduits and pockmarks at the last glacial termination. Modern pore waters contain no methane and there is no negative δ¹³C excursion at the glacial termination that would be indicative of methane or mantle-derived carbon at the time the Δ¹⁴C excursion and pockmarks were produced. An ocean general circulation model equipped with isotope tracers is unable to simulate these large Δ¹⁴C excursions on the Chatham Rise by transport of hydrothermal carbon released from the East Pacific Rise as previous studies suggested. Here we attribute the Δ¹⁴C anomalies and pockmarks to release of ¹⁴C-dead CO2 and carbon-rich fluids from subsurface reservoirs, the most likely being dissociated Mesozoic carbonates that subducted beneath the Rise during the Late Cretaceous. Because of the large number of pockmarks and duration of the Δ¹⁴C anomaly, the pockmarks may collectively represent an important source of ¹⁴C-dead carbon to the ocean during glacial terminations.