Preliminary results and documentation of sediment core CRP-1 from the Ross Sea off Cape Roberts, Antarctica

The first hole of the Cape Roberts Project, CRP-1, was drilled in October, 1997, to a depth of 148 metres below the sea floor (mbsf) before being terminated unexpectedly the loss of fast sea-ice seaward of the rig following a severe storm. The site lies in 150 m of water at 77.008°S and 163.755°E, 16 km off Cape Roberts. This part of the report outlines the geologic setting, a gently tilted sequence near the margin of the Victoria Land Basin, and describes the history of the growth of sea ice, which provided the drilling platform, as well as the history of the drilling itself. Core recovery was around 77% in soft and brittle strata to 100 m and 98% below that. The sequence was found to comprise a Quaternary glacigenic interval down to 43.55 mbsf and below this an early Miocene interval that was also glacigenic. Core properties that were studied include fracture patterns, porosity, sonic velocity and magnetic susceptibility. Velocity in particular was useful in relating the cored sequence to the regional seismic stratigraphy. A preliminary assessment suggests that the bottom of the hole is 15 m short of the boundary between seismic sequences V3 and V4. Analytical facilities new to the Antarctic and used for processing samples for the project are described here and include a bench top palynological processing system and a palaeomagnetic laboratory. The core management and sampling system, which recorded over 2000 samples, is also outlined.

Cenozoic magnetostratigraphy of ODP holes 113-689B and 113-690B

A detailed paleomagnetic study was carried out on biosiliceous and calcareous sediments drilled on Maud Rise, Antarctica, during ODP Leg 113. High-quality APC sections were retrieved in the upper 220 m of Holes 689B and 690B. Average deposition rates range from 3 to 15 m/m.y. A close (25 cm) paleomagnetic sample spacing provided a medium-resolution magnetostratigraphic sequence for the Paleogene and Neogene. Paleomagnetic samples were demagnetized stepwise by alternating fields, and characteristic remanent magnetization directions were derived from detailed vector and difference vector component analysis. A magnetochronologic framework has been established for the first time for the Southern Ocean sedimentary sequences spanning Paleocene to Oligocene and middle Miocene to early Pliocene times. Biosiliceous and calcareous microfossil stratigraphies were used to constrain magnetostratigraphic age assignments. Although average sedimentation rates were rather low, nearly complete sections of the geomagnetic polarity time scale (e.g., Chrons C5 and C5A) could be correlated with the inferred polarity pattern. Miocene and Pliocene records are marked by a high number of hiatuses mainly identified by diatom biostratigraphy. Good paleomagnetic correlation between the two holes is afforded in particular in the middle to upper Miocene. Oligocene magnetostratigraphy reveals a high-quality paleomagnetic record with a mostly complete Oligocene section in Hole 689B at ~5 m/m.y. deposition rate. Hole 690B exhibits higher deposition rates (7-12 m/m.y.), although two hiatuses are present. Early and late Eocene sedimentary sequences could be analyzed in both holes, but in Hole 689B middle Eocene chrons were disrupted by hiatuses and only incomplete polarity intervals C21 and C24 were encountered. Highest resolution (14 m/m.y.) was achieved in Hole 690B in a complete early Eocene and late Paleocene sequence from Chrons C23 to C26, with a number of short polarity intervals detected within Chrons C24 and C25.

Magnetostratigraphy of ODP Leg 104 holes

Results of a detailed paleomagnetic study on largely undisturbed sedimentary sequences recovered in the Voring Plateau region of the Norwegian Sea during Ocean Drilling Program Leg 104 are presented. At each drill site an essentially continuous downhole magnetic reversal pattern could be defined to depths between 200 and 300 m below seafloor allowing correlations with a calibrated geomagnetic time scale and establishing almost complete magnetostratigraphic records for the core material analyzed. A composite section of the drill holes represents the first high-quality chronostratigraphic framework from the lower Miocene through Holocene obtained in the Norwegian Sea. It should provide a basis for first-order correlations with calcareous and siliceous microfossil events and contribute to a further elaboration of the regional paleoceanographic history. A series of major hiatuses in the upper and middle Miocene accounts for about 4 million yr of missing stratigraphic record.

Reefs shift from net accretion to net erosion along a natural environmental gradient

Coral reefs persist in an accretion-erosion balance and ocean acidification resulting from anthropogenic CO2 emissions threatens to shift this balance in favor of net reef erosion. Corals and calcifying algae, largely responsible for reef accretion, are vulnerable to environmental changes associated with ocean acidification, but the direct effects of lower pH on reef erosion has received less attention, particularly in the context of known drivers of bioerosion and natural variability. This study examines the balance between reef accretion and erosion along a well-characterized natural environmental gradient in Kane'ohe Bay, Hawai'i using experimental blocks of coral skeleton. Comparing before and after micro-computed tomography (µCT) scans to quantify net accretion and erosion, we show that, at the small spatial scale of this study (tens of meters), pH was a better predictor of the accretion-erosion balance than environmental drivers suggested by prior studies, including resource availability, temperature, distance from shore, or depth. In addition, this study highlights the fine-scale variation of pH in coastal systems and the importance of microhabitat variation for reef accretion and erosion processes. We demonstrate significant changes in both the mean and variance of pH on the order of meters, providing a local perspective on global increases in pCO2. Our findings suggest that increases in reef erosion, combined with expected decreases in calcification, will accelerate the shift of coral reefs to an erosion-dominated system in a high-CO2 world. This shift will make reefs increasingly susceptible to storm damage and sea-level rise, threatening the maintenance of the ecosystem services that coral reefs provide.