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Stable carbon isotope values of organic carbon have proven useful in identification of the salinity regime of marsh deposits in the Mississippi Delta plain. Using a mixing model and empirical measurements of the ᵹ 13 C of marsh sediments, we show that carbon isotope analysis could be a useful technique on other coastlines. Estimates of the stable carbon isotope values from estuarine deposits along the west and east coast of the United States suggest that fresh and saline sediments each have a characteristic signature. Thus, stable carbon isotope values can provide valuable evidence of sea level fluctuations in these regions. Values of ᵹ 13 C which depart from the predictive model could be caused by diagenetic processes (decomposition) and/or contribution from carbon sources not accounted for in the model. In assessing the relative importance of these factors, we find that complications are minimized when autochthonous carbon from algal production is not an important component of the sedimentary organic matter.