Stuart A. Henrys

Obliquity-paced Pliocene West Antarctic ice sheet oscillations

Thirty years after oxygen isotope records from microfossils deposited in ocean sediments confirmed the hypothesis that variations in the Earth’s orbital geometry control the ice ages, fundamental questions remain over the response of the Antarctic ice sheets to orbital cycles. Furthermore, an understanding of the behaviour of the marine-based West Antarctic ice sheet (WAIS) during the ‘warmer-than-present’ early-Pliocene epoch (∼5–3 Myr ago) is needed to better constrain the possible range of ice-sheet behaviour in the context of future global warming. Here we present a marine glacial record from the upper 600 m of the AND-1B sediment core recovered from beneath the northwest part of the Ross ice shelf by the ANDRILL programme and demonstrate well-dated, ∼40-kyr cyclic variations in ice-sheet extent linked to cycles in insolation influenced by changes in the Earth’s axial tilt (obliquity) during the Pliocene. Our data provide direct evidence for orbitally induced oscillations in the WAIS, which periodically collapsed, resulting in a switch from grounded ice, or ice shelves, to open waters in the Ross embayment when planetary temperatures were up to ∼3 °C warmer than today and atmospheric CO2 concentration was as high as ∼400 p.p.m.v. (refs 5, 6). The evidence is consistent with a new ice-sheet/ice-shelf model that simulates fluctuations in Antarctic ice volume of up to +7 m in equivalent sea level associated with the loss of the WAIS and up to +3 m in equivalent sea level from the East Antarctic ice sheet, in response to ocean-induced melting paced by obliquity. During interglacial times, diatomaceous sediments indicate high surface-water productivity, minimal summer sea ice and air temperatures above freezing, suggesting an additional influence of surface melt under conditions of elevated CO2.

Orbitally induced oscillations in the East Antarctic ice sheet at the Oligocene/Miocene boundary

Between 34 and 15 million years (Myr) ago, when planetary temperatures were 3–4 °C warmer than at present and atmospheric CO2 concentrations were twice as high as today, the Antarctic ice sheets may have been unstable. Oxygen isotope records from deep-sea sediment cores suggest that during this time fluctuations in global temperatures and high-latitude continental ice volumes were influenced by orbital cycles. But it has hitherto not been possible to calibrate the inferred changes in ice volume with direct evidence for oscillations of the Antarctic ice sheets. Here we present sediment data from shallow marine cores in the western Ross Sea that exhibit well dated cyclic variations, and which link the extent of the East Antarctic ice sheet directly to orbital cycles during the Oligocene/Miocene transition (24.1–23.7 Myr ago). Three rapidly deposited glacimarine sequences are constrained to a period of less than 450 kyr by our age model, suggesting that orbital influences at the frequencies of obliquity (40 kyr) and eccentricity (125 kyr) controlled the oscillations of the ice margin at that time. An erosional hiatus covering 250 kyr provides direct evidence for a major episode of global cooling and ice-sheet expansion about 23.7 Myr ago, which had previously been inferred from oxygen isotope data (Mi1 event).

Kinking of the subducting slab by escalator normal faulting beneath the North Island of New Zealand

Seismic reflection imaging shows a marked shallow kink at ∼12 km depth in the Pacific plate beneath the central North Island, New Zealand, that coincides with (1) a decrease in the amplitude of the plate boundary reflection, (2) the locus of prominent landward-dipping splay thrust faults in the overlying plate, and (3) the onset of seismogenesis on the subduction interface and within the subducted plate. We propose that the sharp change in the dip of the plate interface is indicative of the downdip transition from stable to unstable slip regimes. Earthquake focal mechanisms suggest the kinking is accomplished through simple shear on reactivated normal faults in the crust of the subducted plate, akin to the down-stepping motion of an escalator. The geological record of uplift in the overlying plate indicates the escalator has been operating for the last 7 m.y.

Variation of Bottom-simulating-reflection Strength in a High-flux Methane Province, Hikurangi Margin, New Zealand

Bottom-simulating reflectors (BSRs) represent the base of a gas-hydrate zone underlain by widespread free gas. On the southern Hikurangi margin offshore of the east coast of New Zealand, multichannel seismic data reveal that the gas-hydrate province extends from about 600-m (1968-ft) water depth to the Hikurangi Trench and covers an area of about 50,000 km2 (19,305 mi2). We analyzed BSR strength in a grid of seismic data across this area. Simplified rock-physics models were used to estimate the reflection coefficient of BSRs with a gas concentration above which compressional wave velocity is mostly insensitive to gas saturation. This reflection coefficient was found to be 0.20, resulting from at least 8–10% gas saturation. Four percent of the gas-hydrate stability zone on the southern Hikurangi margin is underlain by strong BSRs with reflection coefficients that are 0.20 or stronger. Mapped variations in BSR and sea-floor reflection amplitude ratios and reflection coefficients reveal a strong correlation, on a regional scale, between the amplitude of BSRs and structures that promote fluid flow. Isotope, geochemical, and geophysical data from previous studies onshore point to a thermogenic origin for methane and suggest that New Zealand east coast fluids are derived from accreted, organic-rich, sedimentary sources overlying the subducting slab and that these sources must have an age of about 70 Ma. We therefore speculate that BSR formation on the Hikurangi margin is supported by the long-term recycling of fluids along faults that penetrate through the oldest sediments in the forearc and sole at the plate interface, as mapped in crustal seismic sections elsewhere on the Hikurangi margin. Squeezed subducting sediments at the plate interface may provide a rich source of water driving fluid recycling.

EROSION OF SEAFLOOR RIDGES AT THE TOP OF THE GAS HYDRATE STABILITY ZONE, HIKURANGI MARGIN, NEW ZEALAND – NEW INSIGHTS FROM RESEARCH CRUISES BETWEEN 2005 AND 2007.

It was proposed that erosion of subsea ridges on the Hikurangi margin may be linked to a fluctuating level of the top of gas hydrate stability in the ocean. Since publication of this hypothesis, three field campaigns were conducted in the study area. Here we summarize relevant results from these cruises. We found that water temperature fluctuations occur at lower frequencies and higher amplitudes than previously thought, making it more likely that temperature changes reach sub-seafloor gas hydrates. Dredge samples encountered numerous consolidated mudstones. We speculate that gas hydrate “freeze-thaw” cycles may lead to dilation of fractures in mudstones due to capillary forces, weakening the seafloor. Ubiquitous gas pockets beneath the ridge may lead to overpressure that may also contribute to seafloor fracturing.