New Laboratories, Upcoming Conferences

Radiocarbon dates together with geoarchaeological, soil, and lithic analyses are presented to describe archaeological site 35-CS-9 in Bandon Ocean Wayside State Park, Oregon, northwestern USA. One of the few Oregon middleHolocene coastal sites that includes sediments and artifacts dating to the early Holocene and possibly to the late Pleistocene, it was recorded in 1951 and surface surveyed by archaeologists in 1975, 1986, and 1991, but its depth and antiquity were not tested. In February 2002, we studied the sites stratigraphy and sediments and described 8 strata from the aeolian surface to bedrock at 350 cm depth. Soil samples taken from a cut bank for texture classification, particle size analysis, pH, carbon content, and chemical analysis suggested that the site represented a complete history of Holocene deposits. Excavation of 2 test units in August 2002 uncovered substantial lithic and charcoal remains that confirm a protracted middle-Holocene occupation and suggest that human occupation began in the early Holocene. Charcoal recovered at 235-245 cm dated to 11,000 14C BP, and the deepest lithic artifact was recovered in a level at 215-225 cm. Whether the human occupation was continuous throughout the Holocene, and whether it began in the early Holocene or in the late Pleistocene, can only be determined with further excavations.

As a result of the growing use of multiple geochemical proxies to reconstruct ocean and climate changes in the past, there is an increasing need to establish temporal relationships between proxies derived from the same marine sediment record and ideally from the same core sections. Coupled proxy records of surface ocean properties, such as those based on lipid biomarkers (e.g. Alkenone-derived sea surface temperature) and planktonic foraminiferal carbonate (oxygen isotopes), are a key example. Here, we assess whether 2 different solvent extraction procedures used for isolation of molecular biomarkers influence the radiocarbon contents of planktonic foraminiferal carbonate recovered from the corresponding residues of Bermuda Rise and Cariaco Basin sediments. Although minor ∆14C differences were observed between solvent-extracted and unextracted samples, no substantial or systematic offsets were evident. Overall, these data suggest that, in a practical sense, foraminiferal shells from a solvent-extracted residue can be reliably used for 14C dating to determine the age of sediment deposition and to examine age relationships with other sedimentary constituents (e.g. Alkenones).

The chemical and isotopic compositions of long-chain (C3 6-C3 9) unsaturated ketones (alkenones), a unique class of algal lipids, encode surface ocean properties useful for paleoceanographic reconstruction. Recently, we have sought to extend the utility of alkenones as oceanic tracers through measurement of their radiocarbon contents. Here, we describe a method for isolation of alkenones from sediments as a compound class based on a sequence of wet chemical techniques. The steps involved, which include silica gel column chromatography, urea adduction, and silver nitrate-silica gel column chromatography, exploit various structural attributes of the alkenones. Amounts of purified alkenones estimated by GC/FID measurements were highly correlated with CO2 yields after sample combustion, indicating purities of greater than 90% for samples containing greater than or equal to 100 micrograms C. The degree of alkenone unsaturation (U^(K')37) also varied minimally through the procedure. We also describe a high-performance liquid chromatography (HPLC) method to isolate individual alkenones for molecular-level structural and isotopic determination.

Recent progress in graphite target production for sub-milligram environmental samples in our facility is presented. We describe an optimized hydrolysis procedure now routinely used for the preparation of CO2 from inorganic samples, a new high-vacuum line dedicated to small sample processing (combining sample distillation and graphitization units), as well as a modified graphitization procedure. Although measurements of graphite targets as small as 35 micrograms C have been achieved, system background and measurement uncertainties increase significantly below 150 micrograms C. As target lifetime can become critically short for targets <150 micrograms C, the facility currently only processes inorganic samples down to 150 micrograms C. All radiocarbon measurements are made at the Scottish Universities Environmental Research Centre (SUERC) accelerator mass spectrometry (AMS) facility. Sample processing and analysis are labor-intensive, taking approximately 3 times longer than samples greater than or equal to 500 micrograms C. The technical details of the new system, graphitization yield, fractionation introduced during the process, and the system blank are discussed in detail.

Accelerator mass spectrometry (AMS) determinations of rat bones from natural and cultural sites in New Zealand have produced ages at odds with the accepted date for early human settlement by over 1000 yr. Since rats are a human commensal, this implies either an earlier visitation by people or problems with the reliability of the AMS determinations. One explanation for the extreme ages is dietary variation involving movement of depleted radiocarbon through dietary food chains to rats. To investigate this, we 14C dated fauna from the previously well-dated site of Shag River Mouth. The faunal remains were of species that consumed carbon derived from a variety of environments within the orbit of the site, including the estuary, river, land, and sea. The 14C results showed a wide range in age among estuarine and freshwater species. Terrestrial and marine organisms produced ages within expectations. We also found differences between bone dated using the Oxford ultrafiltration method and those treated using the filtered gelatin method. This implies that contamination could also be of greater importance than previously thought.