Late Pleistocene and Holocene nitrogen cycling in relation to climate, bedrock geology and lake evolution, Central Idaho.

Abstract:

We present two lake sediment isotope records of de-glacial and Holocene nitrogen cycling and productivity from semi arid watersheds in the mountains of central Idaho. Increasing δ¹⁵N profiles from both sediment cores imply increasing nitrogen availability from the middle Holocene to present. Carbon isotope profiles indicate that algal productivity increased concurrently with nitrogen availability. Nitrogen mass accumulation rates (representing a combination of nitrogen delivery and preservation in the sediment) increase along with productivity, suggesting that the increased flux of nitrogen to the lake lead to increasing lake productivity. Increasing precipitation rather than landscape or lake development processes drove the increased watershed nitrogen flux to these lakes over the late Holocene. We rule out landscape and lake development processes because these lakes have different formation times and processes, and occur in watersheds of different lithologies. One of the lakes also appears to have been nitrogen limited over the Holocene (based on sediment composition and sediment carbon nitrogen ratios) and thus sedimentary δ¹⁵N should be similar to δ¹⁵N of watershed dissolved nitrogen. Therefore we attribute increasing δ¹⁵N over the late Holocene to be caused by soil rather than lacustrine cycling/microbrobial processes.