Lance E. Kearns

Evidence for middle Eocene Arctic sea ice from diatoms and ice-rafted debris

Oceanic sediments from long cores drilled on the Lomonosov ridge, in the central Arctic, contain ice-rafted debris (IRD) back to the middle Eocene epoch, prompting recent suggestions that ice appeared in the Arctic about 46 million years (Myr) ago. However, because IRD can be transported by icebergs (derived from land-based ice) and also by sea ice, IRD records are restricted to providing a history of general ice-rafting only. It is critical to differentiate sea ice from glacial (land-based) ice as climate feedback mechanisms vary and global impacts differ between these systems: sea ice directly affects ocean–atmosphere exchanges, whereas land-based ice affects sea level and consequently ocean acidity. An earlier report assumed that sea ice was prevalent in the middle Eocene Arctic on the basis of IRD, and although somewhat preliminary supportive evidence exists, these data are neither comprehensive nor quantified. Here we show the presence of middle Eocene Arctic sea ice from an extraordinary abundance of a group of sea-ice-dependent fossil diatoms (Synedropsis spp.). Analysis of quartz grain textural characteristics further supports sea ice as the dominant transporter of IRD at this time. Together with new information on cosmopolitan diatoms and existing IRD records, our data strongly suggest a two-phase establishment of sea ice: initial episodic formation in marginal shelf areas ∼47.5 Myr ago, followed ∼0.5 Myr later by the onset of seasonally paced sea-ice formation in offshore areas of the central Arctic. Our data establish a 2-Myr record of sea ice, documenting the transition from a warm, ice-free environment to one dominated by winter sea ice at the start of the middle Eocene climatic cooling phase.

Microfeatures of modern sea-ice-rafted sediment and implications for paleo-sea-ice reconstructions

Abstract Distinguishing sea-ice-rafted debris (SIRD) from iceberg-rafted debris is crucial to an interpretation of ice-rafting history; however, there are few paleo-sea-ice proxies. This study characterizes quartz grain microfeatures of modern SIRD from the Arctic Ocean, and compares these results with microfeatures from representative glacial deposits to potentially differentiate SIRD from ice-rafted sediments which have been recently subjected to glacial processes. This allows us to evaluate the use of grain microfeatures as a paleo-sea-ice proxy. SIRD grains were largely subrounded, with medium relief, pervasive silica dissolution and a high abundance of breakage blocks and microlayering. The glacial grains were more angular, with lower relief and higher abundances of fractures and striations/gouges. Discriminate analysis shows a distinct difference between SIRD and glacial grains, with ˂7% of the SIRD grains containing typical glacial microtextures, suggesting this method is a useful means of inferring paleo-sea-ice presence in the marine record. We propose that differences in microfeatures of SIRD and glacial ice-rafted debris reflect differences in sediment transport and weathering histories. Sediment transported to a coastal setting and later rafted by sea ice would be subject to increased chemical weathering, whereas glaciers that calve icebergs would bypass the coastal marine environment, thus preserving their glacial signature.

The Mineralogy of the Buck Hill Syenite Intrusion, Augusta County, Virginia

Buck Hill is a small, elevated ridge located just east of the George Washington National Forest on the 7.5-minute Stokesville Quadrangle, Augusta County, Virginia. In this area, forty-one dikelike intrusions have been mapped (Rader 1969). These intrusives consist of a suite of silica-deficient, alkalic rocks (teschenite, ijolite, and syenite) of Jurassic-Cretaceous age. The local geology is composed of Upper Ordovician, Silurian, and Lower Devonian carbonates and clastics that are regionally folded (Alleghenian Orogeny) and faulted by the Little North Mountain thrust fault (Rader 1969; Johnson, Milton, and Dennison 1971; Fitzgerald 1966). The area intrusives are spatially related to a larger suite of diverse, mafic to alkaline igneous rocks found in Augusta, Highland, and Rockingham counties, Virginia, and bordering Pendleton County, West Virginia (Johnson 1965). These igneous rocks are Upper Cretaceous through Tertiary in age (Fullagar and Bottino 1969; Tso et al. 2003) and represent the youngest igneous activity known in the eastern United States. The exact origin and relationship of these rocks to the regional tectonics is not fully understood. The largest of the intrusive bodies is associated with the positive topographic feature known as Buck Hill (fig. 1). The Buck Hill intrusion is a nepheline-natrolite syenite measuring approximately 100 meters by 300 meters. In-