ALBERT

Description: Pockmarks are variably sized crater-like structures that occur in young continental margin sediments. They are formed by gas eruptions and/or long-term release of fluid or gas. So far no pockmarks were known from the Pacific coast of South America between 51°S and 55°S. This article documents an extensive and previously unknown pockmark field in the Seno Otway (Otway Sound, 52°S) with multibeam bathymetry and parametric echosounding as well as sediment drill cores. Up to 31 pockmarks per square kilometer occur in water depths of 50 to 〉100 m in late glacial and Holocene sediments. They are up to 150 m wide and 10 m deep. Below and near the pockmarks, echosounder profiles image acoustic blanking as well as gas chimneys often crosscutting the 20 to 〉30 m thick glacial sediments above the acoustic basement, in particular along fault zones. Upward-migrating gas is trapped within the sediment strata, forming dome-like features. Two 5 m long piston cores from inside and outside a typical pockmark give no evidence for gas storage within the uppermost sediments. The inside core recovered poorly sorted glacial sediment, indicating reworking and re-deposition after several explosive events. The outside core documents an undisturbed stratigraphic sequence since ~15 ka. Many buried paleo-pockmarks occur directly below a prominent seismic reflector marking the mega-outflow event of the Seno Otway at 14.3 ka, lowering the proglacial lake level by about 80 m. This decompression would have led to frequent eruptions of gas trapped in reservoirs below the glacial sediments. However, the sediment fill of pockmarks formed after this event suggests recurrent events throughout the Holocene until today. Most pockmarks occur above folded hydrocarbon-bearing Upper Cretaceous and Paleogene rocks near the western margin of the Magallanes Basin, constraining them as likely source rocks for thermogenic gas.

Description: The lithium isotopic composition of foraminifera is an established tracer of long-term changes in the global silicate weathering cycle, following the assumption that foraminifera faithfully record the lithium isotopic composition (δ7Li) of seawater. In this study, we demonstrate by utilising benthic foraminifera (Amphistegina lessonii) that were cultured under decoupled pH-[CO32–] conditions, that foraminifera δ7Li is strongly dependent on pH. This is reinforced with δ7Li data from globally distributed core-top samples of Cibicidoides mundulus and Cibicidoides wuellerstorfi, which show the same negative correlation with pH. The dependency of δ7Li on pH is perhaps a surprising result given that lithium speciation in seawater is independent of both pH and carbonate ion speciation. The dependence of lithium incorporation on growth rate was assessed by measuring the calcium isotopic composition; no growth rate dependent incorporation was observed. Instead, we propose that the strength of the 6Li and 7Li hydration spheres (and hence their respective desolvation energy) is pH-dependent, resulting in a significant isotopic fractionation during the incorporation of lithium into foraminifer calcite. The core-top derived δ7Li-pH calibration is used to demonstrate the applicability of this δ11B-independent pH proxy in reconstructing deglacial variations in pH in the South Pacific. The use of foraminifera δ7Li to compliment δ11B-based pH reconstructions has the potential to provide insight into time-dependent variations in porewater/seawater δ11B, temperature and salinity, which were previously unresolvable.

Description: Accelerator mass spectrometry (AMS) radiocarbon dating of ostracod and gastropod shells from the southwestern Black Sea cores combined with tephrochronology provides the basis for studying reservoir age changes in the late-glacial Black Sea. The comparison of our data with records from the northwestern Black Sea shows that an apparent reservoir age of ∼1450 14C yr found in the glacial is characteristic of a homogenized water column. This apparent reservoir age is most likely due to the hardwater effect. Though data indicate that a reservoir age of ∼1450 14C yr may have persisted until the Bølling-Allerød warm period, a comparison with the GISP2 ice-core record suggests a gradual reduction of the reservoir age to ∼1000 14C yr, which might have been caused by dilution effects of inflowing meltwater. During the Bølling-Allerød warm period, soil development and increased vegetation cover in the catchment area of the Black Sea could have hampered erosion of carbonate bedrock, and hence diminished contamination by “old” carbon brought to the Black Sea basin by rivers. A further reduction of the reservoir age most probably occurred contemporary to the precipitation of inorganic carbonates triggered by increased phytoplankton activity, and was confined to the upper water column. Intensified deep water formation subsequently enhanced the mixing/convection and renewal of intermediate water. During the Younger Dryas, the age of the upper water column was close to 0 yr, while the intermediate water was ∼900 14C yr older. The first inflow of saline Mediterranean water, at ∼8300 14C yr BP, shifted the surface water age towards the recent value of ∼400 14C yr.

Description: Highlights • Colder and fresher subsurface waters during glacials in the Central South Pacific. • Depth of source waters forming the AAIW in the CSP shifts between LGM 
and PGM. • LGM-PGM conditions resemble the oceanographic changes caused by SAM. Abstract Southern Ocean Intermediate Waters (SOIWs), such as Antarctic Intermediate Water and Subantarctic Mode Water, play a key role in modulating the global climate on glacial-interglacial time scales. They link the Southern Ocean and the tropics via mechanisms such as “oceanic tunneling” that transport climatic signals across latitudes. Despite their importance, the past evolution of the SOIWs in the Central South Pacific is largely unknown. Here we compare paired Mg/Ca-temperature, stable carbon (δ13C) and oxygen (δ18O) isotope records from surface-dwelling and deep-dwelling planktic foraminifera to infer changes in the water column structure for the last 260 ka in the Central South Pacific (54° S). Our study focuses on the subsurface oceanographic variability controlled by SOIWs, which are subducted at the Subantarctic Front. Our data show that the subsurface ocean in the Central South Pacific was colder and fresher during glacial stages than during the Holocene (0–10 ka BP), suggesting a general glacial enhanced presence of Antarctic Intermediate Water, in agreement with previous studies from the Eastern Equatorial Pacific and the Southeast Pacific. However, the subsurface ocean salinity differs for both glacial stages, with fresher condition during the Last Glacial Maximum (LGM; ∼26.5–19 ka BP) and more saline condition during the Penultimate Glacial Maximum (PGM; ∼155–140 ka BP). The δ13C data also show contrasting conditions for both glacial time windows in the upper water column, with a large depletion of 0.37‰ in δ13C from the LGM values, suggests a larger contribution of “old” low δ13C deep waters at intermediate depths at the study site during the PGM, plausibly due to stronger upwelling in high southern latitudes. The dissimilar scenarios between the LGM and the PGM may have been caused by processes that are analogous to the phase-switch in modern day Southern Annular Mode.

Description: During the last deglaciation, the opposing patterns of atmospheric CO2 and radiocarbon activities (Δ14C) suggest the release of 14C-depleted CO2 from old carbon reservoirs. Although evidences point to the deep Pacific as a major reservoir of this 14C-depleted carbon, its extent and evolution still need to be constrained. Here we use sediment cores retrieved along a South Pacific transect to reconstruct the spatio-temporal evolution of Δ14C over the last 30,000 years. In ∼2,500–3,600 m water depth, we find 14C-depleted deep waters with a maximum glacial offset to atmospheric 14C (ΔΔ14C=−1,000‰). Using a box model, we test the hypothesis that these low values might have been caused by an interaction of aging and hydrothermal CO2 influx. We observe a rejuvenation of circumpolar deep waters synchronous and potentially contributing to the initial deglacial rise in atmospheric CO2. These findings constrain parts of the glacial carbon pool to the deep South Pacific.