This project answers a simple question: why are there so few fossils in sediment cores from Antarctica?s continental shelf? Antarctica?s benthos are as biologically rich as those of the tropics. Shell-secreting organisms should have left a trail throughout geologic time, but have not. This trail is particularly important because these organisms record regional climate in ways that are critical to interpreting the global climate record. This study uses field experiments and targeted observations of modern benthic systems to examine the biases inflicted by fossil preservation. By examining a spectrum of ice-affected habitats, this project provides paleoenvironmental insights into carbonate preservation, sedimentation rates, and burial processes; and will provide new approaches to reconstructing the Cenozoic history of Antarctica. Broader impacts include graduate and undergraduate research and education, development of undergraduate curricula to link art and science, K12 outreach, public outreach via the web, and societal relevance through improved understanding of records of global climate change.
Because it is the repository (as ice) of most of the world’s freshwater and is sensitive to climate change, predicting Antarctica’s response to global change is important. Reconstructing the paleoclimate of the last ~ 5 million years provides clues about the future but is difficult because, other than the ice itself, there are scant records of the history on the continent. Luckily, however, seafloor sediments around Antarctica provide a record. Analyses of cores of the sediments reveal repeated advances and retreats of icesheets in response to climate change. Studies of present-day Antarctic climate indicate that the extent of sea ice, a thin layer of floating ice that forms when the ocean freezes, and the amount of multi-year sea ice (that does not melt each year) exert control on ocean-atmosphere interaction and thus on climate. However, recognizing the presence of multi-year sea ice and determining its extent in the past is hampered by lack of knowledge of 1) the characteristics of the sedimentary sequence under multi-year sea ice including the grain size of sediments, the physical sedimentary structures, and the biogenic structures produced by animals living on the sediment, and 2) the processes by which this sequence forms. The goal of this project was to document the characteristics of the sedimentary sequence under multi-year sea ice in Explorers Cove (EC) at the mouth of the ice-free Taylor Valley and to investigate the formational processes. Using methods that included 1) SCUBA diver coring of seafloor sediments, quantitative assessment of animals and biogenic structures, deployment of sediment traps and experimental arrays to determine rates of calcite skeletal dissolution and of sediment disruption by animals, as well as 2) laboratory study of animal effects on sediment and grain size analysis of seafloor, sea ice, and Taylor Valley sediment, salient characteristics and important processes were identified. Characteristics: The sedimentary sequence on the seafloor underneath multiyear sea ice consists of sand that is similar in size to its onshore source, and is structureless (e.g., lacks layering and discrete burrows formed by animals). Calcareous skeletal elements of megafaunal animals are rare. Sediment is coarsest grained near the delta, becoming finer both laterally away from the delta and farther offshore. Significance: The structureless and fossil-poor sand with no discrete biogenic structures that accumulates on the nearshore seafloor of EC resembles massive sands lacking fossils that form under floating ice shelves, particularly near the grounding line. Sedimentary deposits similar to those in EC will occur in many multi-year sea ice regimes in which sediment is delivered by delta distributaries or wind rather than by ice to a seafloor colonized by active animals. This demonstrates that massive sands retrieved in cores originate in at least two different settings (under multi-year sea ice and near grounding line of ice sheet) with very different implications for paleoclimate. Processes: How is sediment transported? 1) During the short summer small meltwater streams form deltas (Fig. 1) whose distributaries flow under the sea ice and immediately drop their sediment load, making a steep slope (Fig.2); rate of deposition drops from >4 - <0.02 cm yr-1 from shore to 50 m offshore. 2) Foehn winds that blow seaward through Taylor Valley (Fig. 3) transport dark sediment onto the sea ice (Fig. 4). Although near the shore the sea-ice sediment is coarser than the underlying seafloor sediment, by 2 km offshore, seafloor and sea ice sediment are similar. However, sea ice sediment has fewer fines than either its source in the Taylor Valley or the seafloor sediment; perhaps they are blown 10’s to 100’s of km offshore. What animals live on the seafloor and what do they eat? The dominant animals are the brittle star Ophionotus victoriae and the Antarctic scallop, Adamussium colbecki. The food source is ice algae that live within the sea ice and fall to the seafloor. How is layering of the sediment destroyed? The scallops "clap" their shells, resuspending the sediment for food and forming divots (Fig. 5) (www.youtube.com/watch?v=FeUilWF-MoI). Mean EC number of scallops (1.7 m-2) resuspending sediment as observed in laboratory (45 cm3 d-1) rework a layer 2.7 cm thick in a year, greatly exceeding sedimentation rate. What happens to calcareous shell material? It dissolves in the cold water. In this study, ophiuroid ossicles suspended in seawater in EC for two years dissolved significantly. This demonstrates why the thousands of meters of Cenozoic sediments in core recovered from drilling projects around Antarctica lack skeletal material. Sand delivered to the coast by streams and blown onto sea ice accumulates under multi-year sea ice (Fig. 6). The abundant animals homogenize the sediment, but their skeletal remains dissolve, removing evidence of the biologic imprint on the sedimentary record. Miller gave presentations about this study to ~1,000 people, in classes and at museums, senior centers, and pre-schools. Three undergraduates, all pursuing graduate studies, were supported.
