delta15N in mollusk shells as a potential paleoenvironmental proxy for nitrogen loading in Chesapeake Bay.
详细信息
- 作者：Black ; Heather Dawn.
- 学历：Master
- 年：2014
- 毕业院校：The University of Alabama
- Department：Geology.
- ISBN：9781321232165
- CBH：1566158
- Country：USA
- 语种：English
- FileSize：25338478
- Pages：89
文摘
Crassostrea virginica is one of the most common oyster species in North America and is frequently found in archaeological sites and sub-fossil deposits,especially in the eastern US. Although there have been several sclerochronological studies on delta13C and delta 18O in the shells of this species,little is known about delta 15N stored within the shells,which could potentially be a useful paleoenvironmental proxy to determine nitrogen loading and the subsequent anthropogenic impacts within an area. In order to potentially serve as paleoenvironmental proxies for N loading,bivalve shells organic matter needs to remain chemically unaltered. Since ancient peoples cooked most archaeological shells before depositing them in shell middens,it is necessary to determine if prehistoric cooking methods alter either %N or delta15N stored within the shells. Twenty C. virginica oysters and twenty-two Mercenaria spp. clams were treated to five different prehistoric cooking methods: direct exposure to hardwood coals,roasting above hardwood coals,roasting in a dry oven,boiling in freshwater,or boiling in seawater. Each shell was bisected through the resilifer with one half treated with one of the five prehistoric cooking methods and the remaining half serving as a control. With the exception of roasting above the hardwood coals,prehistoric cooking methods do not significantly alter either %N or delta15N within the shells. Those shells roasted above the coals were typically enriched in both %N and delta 15N,which is likely an effect of smoke coming from the hardwood coals and infiltrating pore spaces within the outer layers of the shell. Ninety archaeological C. virginica shells ranging in age from ~120 to 3,400 years old and thirty-two modern C. virginica shells were collected in Chesapeake Bay at the Smithsonian Environmental Research Center in Edgewater,Maryland. One valve from each shell was sub-sampled and the calcite powder was analyzed without acidification pretreatment) using an EA-IRMS system equipped with a CO2 trap to determine the %N and delta15N content of the shells. Comparison of %N and delta15N in C. virginica shells from the six different time periods studied show relatively constant values from ~3,400 years ago to 1820 AD. Between 1820 and 1890,there are rapid increases in both %N and delta15N in the shells,which continue to exponentially increase in value to the modern shells. The increases in %N and delta15N are correlated with increased anthropogenic impact due to human population,sewage discharge,and urbanization in Chesapeake Bay at this time. Therefore,it is likely that C. virginica shells can be used as a paleoenvironmental proxy to measure the anthropogenic impact of a specific area over time. However,the constant,relatively low %N and delta15N values from ~3,400 years ago to 1820 AD compared to the increased N concentrations and enriched delta15N shells from the modern periods could be influenced by diagenetic alteration of the shell after burial in the midden. It is possible that the shells are losing N and preferentially losing 15N over time. More research is necessary to determine if bivalve shells are geochemically stable with regard to N over time or if diagenesis is likely to have occurred in these shells.