Susan Kidwell, University of Chicago Clark Alexander, Skidaway Institute of Oceanography
Biologists increasingly appreciate the need for longer term perspectives on the presence and abundance of species in coastal environments in order to evaluate how these important ecosystems have been altered by human activities, which in most areas began well before the advent of quantitative surveys in the early 20th century and federally mandated biomonitoring in the 1970s. Very young fossil records ? that is, skeletal remains from the last few decades, centuries, and tens of millennia, embedded within the upper few meters of the sedimentary record ? offer a promising means both of reconstructing changes in ecosystems over the full history of cultural/industrial development and of acquiring the information on truly natural conditions that is essential to developing restoration targets. However, to confidently merge these geologically young paleontological data with conventional neo-biological data, we need to understand how, if at all, biological information is altered with the progressive, natural burial of shelly remains. Research over the last 20 years finds that molluscan death assemblages (clam and snail shells) present within the uppermost, storm- and animal-stirred few centimeters of the seabed can include some fairly old shells (thousands of years old, especially on open continental shelves), but are strongly dominated by individuals that died during the last (most recent) ?urban? century and especially the last few decades. Death assemblages from this mixed layer are thus clearly time-averaged, but the strongest signal is from the final stages of skeletal accumulation, much like a photographic time-exposure produces a somewhat blurry track of a moving object whose final position is captured with snapshot-like clarity. Here, with NSF funding, we will test for the first time the extent to which skeletal assemblages retain this level of temporal resolution as they undergo progressive burial into deeper, more permanent layers of the Holocene sedimentary record, where the assemblage is no longer refreshed by new shell input and is subject to different chemical regimes. We will leverage extensive prior knowledge of seafloor animals and environmental conditions on the southern California continental shelf to conduct a series of observational experiments on the key factors thought to control skeletal accumulation, namely (1) the intrinsic durability of shell material (focus on six key genera of bivalves, differing in shell mineralogy and microstructure), (2) rates and types of sediment accumulation (land-derived mud and sand, plus shell-rich seabeds deprived of sediment), and (3) rates and depths of sediment mixing by storms and burrowing animals, with factors 2 and 3 quantified using radioisotopic methods. Shell ages will be based on radiocarbon-calibrated amino-acid racemization dating, which we have already determined works well in this region. Data on how temporal resolution changes with burial will be generated by dating sets of shells from a series of horizons in long (3-4 m) sedimentary cores. We will take these cores at sites both along the known legacy gradient of sediment toxicity from DDT-charged wastewater effluents on the Palos Verdes shelf and from historically uncontaminated seafloors, providing an ?unnatural? experiment on the effects of bioturbation. Using stochastic simulations and likelihood analysis of shell-age frequency distributions as well as conventional statistical analysis of experimental results, this project will identify the dominant mechanisms that determine shell preservation, which is relevant to the ecological fidelity as well as the temporal resolution of skeletal assemblages in both the deep- and near-time fossil records. Of even broader implications, we will develop and refine protocols for merging biological information from paleontological and neo-biological sources, as part of the burgeoning new field of conservation paleobiology. Research papers will be published in marine biological and pollution journals rather than only earth science journals to promote scientific integration. We already have a cooperative relationship with scientists from several regional agencies charged with environmental management, who will be kept up to date with research results by talks on site. Archived skeletal assemblages from our cores will constitute a new resource for future studies of diverse types (e.g., morphology, sclerochronology, isotopes, molecules), particularly when combined with extensive mixed-layer death assemblages that we have already archived from past (1970s) and ongoing (2003 ? present) surveys and biomonitoring of living shelf communities. The project will provide field and lab experience for a post-doctoral scholar and for undergraduate and graduate student assistants drawn from an historically black institution (Savannah State). We will also develop a lab exercise for use in local high schools and 2-year or 4-year colleges on the historical ecology of the southern California marine bight and the power of sedimentary cores for environmental and ecological analysis. This region has strong public interest and governmental commitment to coastal environmental quality, and thus has a good existing infrastructure for public outreach.
The major goal of the sedimentary process and stratigraphic work in this project is to support investigation into the paleoecological and taphonomic questions being asked by my co-PI, Sue Kidwell (see Kidwell Outcomes Repot for details). In addition, I have used these data to develop a regional overview of sedimentary processes in a portion of the Southern California Bight extending from Pt. Dume to Oceanside, through the distribution of 100-y timescale accumulation rates, the rates of biological sediment mixing, and textural parameters. These data are used to determine the dynamics of sediment transport and deposition within this portion of the Bight, building on information provided by Alexander and Venherm (2003) and Alexander and Lee (2010). Accumulation Rates Across the Santa Monica, San Pedro and Oceanside margins, 210Pb accumulation rates decrease in an offshore direction from 0.04-0.56 g cm-2 y-1. This is consistent with earlier studies in the Southern California Bight where rates ranged from 0.02-1.71 g cm-2 y-1 and showed similar patterns. Rates are highest in nearshore areas and rapidly decrease across the shelf; the decrease across the slope, rise and basins is slower, suggesting that offshore transport of wave-resuspended material occurs as relatively dilute nepheloid layers and that hemiplegic sedimentation dominates the supply of sediment to the outer shelf, slope and basins. In general, the shelf narrows and the continental slope becomes steeper toward the south in response to tectonic forcing, allowing fluvial sediment to more easily escape the shelf to slope and offshore basin sinks. Patterns of sedimentation and concepts for sediment offshore transport follow the model suggested by Warrick and Farnsworth (2009). In that model, when canyons do not exist off river mouths, narrow margins with critical slope shelves and little sediment supply tend to move fluvial sediments across and off the shelf, leading to lack of a shelf mud belt and the dominance of hemipelagic sedimentation along the margin. Excess 210Pb profiles exhibit steady state characteristics in most areas, except in the axes of the submarine canyons and where seafloor channels linked to these canyons are observed, where non-steady-state 210Pb profiles are observed. X-radiographs of shelf and slope cores are typically mottled, documenting typical levels of biological mixing for these settings; canyon and canyon distributary channel cores exhibit laminated beds documenting rapidly deposited sediments associated with down canyon flows. Mixing rates and depths derived from excess 234Th profiles suggest that depths (2-6 cm) and rates (4.8-34.7 cm2 y-1) of mixing are consistent with earlier studies of the Southern California bight (1-5 cm and 0.0-49.9 cm2 y-1). Grain Size Unimodal sediments fine with distance from the inner shelf to offshore basins from ~1 Φ to ~8 Φ. Sediments fine rapidly across the shelf from shore to ~100 m water depth from sands to silts. The gradation from silts to clays across the slope out into the basins in much more gradual, again suggesting that hemipelagic sedimentation processes dominate beyond the shelf. Grain size of long (~6m) cores from the Orange County and Malibu shelves exhibit relatively consistent sand, silt and clay components, suggesting that inputs have been relatively constant over the past few thousand years; sediments in the long cores near Palos Verdes reflect the changing input that have been present in that area with time, exhibiting dramatic shifts in grain size, reflecting input from fluvial sources, proximal landslide input, and anthropogenic contributions. Sediment Budget A preliminary budget for the Southern California Bight based on these data suggests that the inputs to the region need to be better defined to approach this issue. At present, estimates suggest that about 2 times more sediment is accumulating in the margin than can be accounted for from current sources. Most importantly, the contribution of material from cliff erosion is poorly quantified and varies by coastal segment in the study area; Alexander is currently working toward addressing this issue. Integration with Taphonomic Studies The age structures of the boxcores have been determined by 210Pb to assign discrete ages down core. These time horizons provide a window into past community structure in the upper seabed in the Southern California Bight. These observations serve to a) ground truth the existing collection of live/dead assemblages collected by water management districts over the past 40 years , b) provide a way to translate from the collections to the preserved record, c) extend our knowledge of community structure into the historic past, and 3) provide a critical linkage to changing communities over the past few thousand years. Results from integrating our studies illustrate the power of our approach – we have demonstrated the ability to track the rise and fall of chemosymbiont-bearing bivalve communities with the introduction and increase of nutrients from an adjacent sewage outfall on the Palos Verdes shelf.
