¹⁴C Dating of Plant Macrofossils in Lake Sediment

Abstract

ABSTRACT. Macrofossils of terrestrial plants have been picked from a sediment core taken in Lake Lobsigen, a small lake on the Western Swiss Plateau. The sediments were previously analyzed for pollen composition, plant and animal macrofossils, and stable isotopes. Plant macrofossils were selected near pollen zone boundaries in Late Glacial and early Postglacial sediment for 14C dating by AMS. In the same lake carbonate and gyttja (aquatic plant) samples were dated by decay counting. The dates on terrestrial material are generally younger than those on carbonate and gyttja, ie, material reflecting the 14C/C ratio of dissolved bicarbonate in lake water. This is probably due to a contribution of dissolved limestone carbonate and thus a somewhat reduced 14C/C, ratio in the lake's water (hard water effect).

Varve, paleomagnetic, and 14 C Chronologies for late Pleistocene events in New Hampshire and Vermont (U.S.A.)

“…Bulk organic samples from the bottoms of lake cores avoid problems associated with marine samples such as marine reservoir variations (Mangerud, 1972;Hjort, 1973;Mangerud and Gullikson, 1975;Bard, 1988;Bard et al, 1994;Birks et al, 1996) and the influences of meltwater in the marine environment (Sutherland, 1986;Hillaire-Marcel, 1988;Rodrigues, 1992). However, lacustrine bulk sediment samples can also yield anomalous old ages for other reasons (Shotton, 1972;Oeschger et al, 1985;Andrée et al, 1986;Wohlfarth, 1996). Aquatic plants as well as other aquatic organisms, which are frequently a significant part of bulk organic samples from lake cores, acquire carbon from lake water in which the concentration of 14 C may be lower than the atmosphere.…”

“…However, comparing bulk 14 C ages with an existing lake bottom chronology that is also based on bulk sediment ages does not allow us to recognize more subtle errors (<1000 yr) that may be pervasive. This can only be accomplished with a direct comparison of bulk sediment and plant macrofossil ages, which have systematically been different by 300-800 yr in European lake sediment studies (Oeschger et al, 1985;Andrée et al, 1986;Wohlfarth, 1996). Although it is possible for a bulk sediment sample to yield an accurate atmospheric 14 C age, systematic testing for subtle errors has not been incorporated into any chronology in New England.…”

“…In addition to aged water from glacial melting, baseflow, and older organic sediment (Abbot and Stafford, 1996), the bedrock may have provided a source of carbon that could create anomalous 14 C ages for freshwater bodies. Bulk samples of organic lacustrine sediment, especially gyttja, have consistently yielded anomalous ages as compared to terrestrial plant macrofossils in attempts to calibrate the Swedish Varve Chronology (Wohlfarth et al, 1993) and other lacustrine records (Oeschger et al, 1985;Andrée et al, 1986). The 12.9 ka 14 C age for varve 6156 is older than the three other 14 C ages from NE varve 6150 at Canoe Brook (Table I) by about 450-600 14 C yr (about 1200 varve or calibrated yr, Fig.…”

On the reconstruction of palaeo-ice sheets: Recent advances and future challenges

“…This is due to problems such as dissolved carbonates in water or detrital carbon reworked from carbon-bearing rocks and sediments. In Switzerland, for example, Andree et al (1986) found an 800-year offset in radiocarbon age between bulk sediment and AMS plant macrofossil ages for the late-glacial interval, while offsets of up to 8 ka occur in basal lake sediments in central North America (Grimm et al, 2009). In another example, the initiation of 13 lakes/bogs across a landscape previously covered by the south-eastern LIS provided a tight cluster of AMS ages between 16 and 15 ka (Peteet et al, 2012), which is 5-9 ka later than the time of ice retreat based upon the extrapolation of bulk chronologies (Dyke et al, 2003), varves (Ridge, 2004), and cosmogenic dates (Balco et al, 2009).…”

Using the N/C ratio to correct bulk radiocarbon ages from lake sediments: Insights from Chilean Patagonia

“…Although the hardwater effect is relatively well understood (e.g., Andrée et al, 1986 , 1991;Barnerkow et al, 1998;Grimm et al, 2009), the processes involving transport and incorporation of soil organic matter in aquatic systems, and the effect of soil organic carbon on bulk radiocarbon ages in general, remains poorly constrained (e.g., Eglinton, 2010). The few authors that address the issue conclude that bulk radiocarbon ages are not accurate enough to provide reliable core chronologies (Björck and Håkansson, 1982;Björck et al, 1998;Abbott and Stafford, 1996) but they do not propose a method to correct ages obtained on such samples, although Björck and Håkansson (1982) noticed that the age difference is related to the amount of re-deposited pollen grains and inversely related to the total organic carbon content (TOC) of the sediment.…”