NITROGEN ISOTOPES AS PROXIES FOR PALEOENVIRONMENTAL CHANGES IN ESTUARINE SYSTEMS

The sedimentary archive from coastal bays and estuaries may contain important records of past environmental and climate changes. Recent studies from Copano Bay, TX have shown evidence of a gradual and sometimes punctuated transgression that occurred along most of the estuaries along the northwestern Gulf of Mexico and globally over the past 9 ky. We used this well documented history to determine whether nitrogen isotopes can be used as a proxy for paleoclimatic or environmental changes.

We focused our investigation on the core tops of six cores and the full length of two cores taken along a dip-oriented transect through Copano Bay. We measured nitrogen isotopic compositions at approximately 0.5 m intervals through the two cores that reach depths up to 16 m below the bay floor. Radiocarbon ages where used to determine the timing of nitrogen isotopic shifts within the sedimentary record. The δ¹⁵N values vary along the transect reflecting the range of environmental conditions in the bay. Data from the more landward core indicates a distinct shift in δ¹⁵N at approximately 7.5 kya that corresponds to a shift from deltaic to upper-bay deposits. We see a similar shift in the δ¹⁵N from the more seaward core corresponding to the change from an upper-bay to an open-bay environment that occurred at approximately 8.2 kya. The results indicate that changes in sedimentary δ¹⁵N profiles correlate strongly with known environmental shifts, and thus can potentially be used as a paleoenvironmental proxy within estuarine sediments.