ALBERT

Description: The main purpose of cruise JR298 was to collect marine geological and geophysical samples and data to support International Ocean Discovery Program (IODP) proposal 732- Full2, “Sediment drifts off the Antarctic Peninsula and West Antarctica” (Channell, Larter, Hillenbrand et al.). The ship time was allocated for this purpose on the basis of a Site Survey Investigation grant from the NERC UK-IODP Programme (NE/J006548/1: Depositional patterns and records in sediment drifts off the Antarctic Peninsula and West Antarctica) to R.D. Larter, C.-D. Hillenbrand (both BAS), D.A. Hodell (University of Cambridge) and A.G.C. Graham (University of Exeter). The data and samples collected will also be used in two Collaborative Gearing Scheme projects, an Antarctic Science Bursary project, a University of Cambridge PhD studentship, and within the National Capability remit of the BAS Science Teams in “Geology and Geophysics” and “Palaeoenvironments and Climate Change”. These projects are: • Tracing and reconstructing the neodymium and carbon isotopic composition of circum-Antarctic waters (CGS-100, PI: A.M. Piotrowski, Department of Earth Sciences, University of Cambridge; cruise participants: A.M. Piotrowski and T.J. Williams). • Structural characterisation of Late Quaternary sediments from West Antarctic contourite drifts using three dimensional X-ray imaging (CT-scanning) (CGS-98, PI: C. Ó Cofaigh, Department of Geography, Durham University; cruise participant: J. Horrocks) • Tracing the Quaternary evolution of the Antarctic Peninsula and West Antarctic Ice Sheets using lead isotopes in ice-rafted feldspar mineral grains (Antarctic Science Bursary awarded to C. Cook). • Seismic imaging of oceanographic structures and processes in the Southern Ocean south of the Polar Front (component of University of Cambridge/BP Institute PhD studentship; primary supervisor: N.J. White, Department of Earth Sciences, University of Cambridge; PhD student and cruise participant: K.L. Gunn). • Modelling crustal structure across the Bellingshausen Gravity Anomaly and oceanic fracture zones formed at the Antarctic-Phoenix Ridge through integration of marine potential field and seismic data (Collaboration between two BAS Science Teams; cruise participant: T.A.R.M. Jordan). 2 The cruise also provided support for physical oceanography projects by deploying six Argo floats and rescuing a malfunctioning sea glider. Towards the end of the cruise, RRS James Clark Ross was diverted to Rothera to uplift 16 personnel who had been flown across from Halley in two ALCI Basler aircraft because the sea ice situation in the Weddell Sea was considered to pose a significant risk to the scheduled last call of the season at Halley by RRS Ernest Shackleton. This uplift resulted in a two-day delay to arrival at Punta Arenas at the end of the cruise, which was in addition to a two-day extension already agreed as a result of departure from Punta Arenas having been delayed by slow refuelling. Adverse weather conditions, particularly during the first half of the cruise, resulted in more downtime than the amount of contingency time that had been allowed in the proposal. As a result, one less piston core and about 20% fewer line-km of seismic data were collected than had been planned. Nevertheless, the key objectives were achieved and the cores and data that were collected are of very good quality. The data and cores collected on cruise JR298, combined with existing data and cores, should satisfy all of the requirements of the Site Characterisation Panel and the Environmental Protection and Safety Panel of IODP. They will also provide a good basis for addressing the science objectives set out in the UK-IODP Site Survey Investigation proposal and those of the ancillary projects listed above.

Description: Understanding of Cretaceous plate motions, and closure of the global plate circuit, is hampered by the "Cretaceous Normal Superchron” (CNS) between 120 and 84 Ma, an interval of very stable geomagnetic field that produced no magnetic striping over the ocean floor. This coincided with eruption of several oceanic large igneous provinces (LIPs), huge outpourings of magma (〉 100,000 km3) that created vast regions of volcanic and related rocks (Coffin and Eldholm, 1993), concentrated in time in what is called a superplume event, and associated with a global episode of plate reorganisation (Matthews et al., 2012). Lack of sea floor magnetic striping during the CNS has complicated the task of reconstructing plate motions in detail, leaving large uncertainties in the relative positions of continents, oceans, and the LIPs they bear. Perversely, prolonged stability of the geomagnetic field at these times means that sea floor magnetic striping, which is normally used to reconstruct plate motions, does not form when changes in plate motions can be at their greatest. 123 million years ago, one of the largest LIPs in Earth history was erupted in the palaeo-Pacific Ocean (Chandler et al., 2012; Taylor, 2006) at the beginning of the CNS, the 5 million km2 Ontong Java-Nui super-plateau (OJN; 2/3 the area of Australia). Shortly afterwards, as a result of the changes in palaeo- Pacific plate configuration associated with its formation, it is thought to have split into at least three component parts that are recognised today (e.g. Fig. 1) (Taylor, 2006), the Ontong-Java, Manihiki and Hikurangi plateaus (Fig. 1). 105 million years ago, still within the CNS, a global reorganisation of plate motions, associated with mountain building events on continental margins, was triggered by changes in subduction along the Gondwana margin in the vicinity of West Antarctica (Matthews et al., 2012). Matthews et al. (2012) presented two hypotheses for this to account for global plate reorganisation: 1) increased ridge crest–trench interaction at subduction zones; or 2) collision of the Hikurangi Plateau fragment of the OJN with the Gondwana margin (Fig. 1). The authors favoured their first hypothesis because Hikurangi Plateau collision, they argued, does not affect a sufficient length of the subducting margin to trigger plate motion changes. However, it has been observed (Taylor, 2006) that a further large piece of the OJN may have rifted-off the unusually straight Manihiki Scarp (Fig. 1) and been removed to the southeast. Preliminary plate motion modelling suggests that a LIP fragment derived from the Manihiki Scarp would have collided with the Gondwana margin in the vicinity of the Antarctic Peninsula. A candidate feature for evaluation has been identified near the southern end of the peninsula. Here, a high amplitude rectilinear aeromagnetic anomaly off the west coast, the Charcot Anomaly (CA) (Johnson and Ferris, 1997), is over 1000 km long (Golynsky et al., 2002) and represents one of the longest unexplained linear continental margin anomalies on Earth. Inboard of this anomaly lies the latitudinally restricted and enigmatic collisional-accretionary Palmer Land Event orogeny, the origin of whose second phase at 103–100 Ma remains unexplained (Vaughan et al., 2012). These data hint that the CA may be the geophysical expression of the trailing edge of this fragment of the OJN (Labelled “?CAP” on Fig. 1) and that the second phase of the Palmer Land Event orogeny may be the on-land expression of its collision with, and partial subduction beneath, the Gondwana margin 100 million years ago.