JOHN T. ANDREWS, Institute of Arctic and Alpine Research and Department of Geological Sciences, University of Colorado, Boulder, Colorado 80309
A.J.T. JULL, National Science Foundation, Arizona Accelerator Mass Spectrometry Facility, University of Arizona, Tucson, Arizona 85721
AMY LEVENTER, Department of Geology, Colgate University, Hamilton, New York 13346
The West Antarctic Ice Sheet Project (WAISP) is a multidisciplinary effort concerned with developing past, present, and future scenarios for the history of this marine-based section of the antarctic ice sheet (Bindschadler 1991, 1995). A major issue is how to develop comparable time-histories for a variety of proxy records including
In particular, the problem with dating marine sediments around the antarctic continental margin is that, frequently, marine carbonates are scarce to nonexistent, and dates must be obtained on the acid-insoluble organic fraction. Such samples from the modern seawater/sediment interface give ages between 2,000 and 5,000 carbon-14 (14C) years (Domack et al. 1989; DeMaster, Ragueneau, and Nittrouer 1996; Harris et al. 1996; Licht et al. 1996). Why is this? We can deduce that it is probably associated with the recycling of old carbon (Truswell and Drewry 1984), but geographical and temporal variations are not well constrained.
This short article is concerned with one problem in dating surface marine sediments. Several sources of variability might affect the radiocarbon date of surface marine samples (Andrews et al. in preparation):
Another concern, which is rarely if ever addressed, however, is within-sample variability: the problem of replicating a result from samples collected a few to tens of centimeters apart. Normally, we would obtain a date from a single sample and then assume that if we repeated the experiment, 95 percent of the time the resulting age would lie within 2 standard errors about the mean. For example, a date of 2,050±65 before present implies that we are 95 percent sure that the true age lies somewhere between 1,920 and 2,180 years ago.
Our data consist of three sets of three dates and a single acid-insoluble/marine carbonate pair (table). We used three surface samples from cruises NBP 9606, collected by Leventer, and a set of samples from a core ( NBP 9501-39), sampled by Licht.
The samples were shipped to the Accelerator Mass Spectrometry (AMS) National Science Foundation Facility at the University of Arizona. Sample treatment was carried out at the AMS Facility and consisted of an acid pretreatment to remove any carbonate fractions. The sample is then washed with distilled water and dried. The samples are further processed as discussed in the next paragraph.
At the AMS Facility, all pretreated samples were combusted with copper oxide to make carbon dioxide (CO2). The CO2 gas is cleaned by passing the gas over zinc at room temperature and copper and silver at 600°C. The gas is then split and a portion reserved for stable-isotope measurement by mass spectrometry. The larger split is further processed to graphite using the methods described by Donahue, Jull, and Linick (1990). The graphite powder is pressed into a target holder and mounted in a 32-position carousel, which is placed in the ion source of the AMS. The calculation procedures are reported in Donahue, Linick, and Jull (1990).
The nature of the general problem is identified immediately when comparing the marine carbonate and acid-insoluble fraction (aif) from NBP 9606-38 (table). The brachiopod date of 1,065±45 is somewhat younger than the generally accepted ocean reservoir correction of 1,200 to 1,300 years before present (Gordon and Harkness 1992; Berkman and Forman 1996). This date, however, is nearly 2,500 years younger than the aif age of 3,580±50. An examination of the two surface-data sets ( NBP 9606-91 and -15) indicates that the maximum range in the reported surface ages is 170 years and 480 years, respectively. In NBP 9606-15, the three samples have overlapping error bars at the 95 percent confidence level, but in NBP 9606-91, the maximum difference of 480 years is larger than would be expected by chance, suggesting some lateral variability in sediment organic composition. Note that the isotopic carbon-13 (d13C) values differ between NBP 9606-15 and NBP 9606-91 and NBP 9501-39 (table). The three replicate samples from the core at a depth of 98-100 centimeters cluster tightly together and have a maximum range in quoted values of only 145 years .
In conclusion, our data suggest that some lateral variability in sediment organic composition exists and could add a small variation in sample age but that this variability is small compared to the 14C age of the surface sediments.
This research has been supported by grants from the National Science Foundation, namely OPP 96-14287 (Andrews), EAR 95-08413 (Jull), and OPP 97-96266 (Leventer).
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