The Development And Application Of Stable Oxygen And Hydrogen Isotope Analyses Of Chironomidae (Diptera) As Indicators Of Past Environmental Change

Abstract

Environmental change continues to be of increasing interest to scientists in all disciplines, and there is a paramount need to gain a comprehensive understanding of the impacts of environmental change in the past to better predict the future. A challenge associated with interpreting past change is a lack of reliable proxies to infer past environmental conditions preserved in the fossil record. My research has been dedicated to developing techniques associated with analyzing the stable oxygen and hydrogen isotopic composition (delta18O and deltaD) of subfossil chironomid (Chironomidea: Diptera) headcapsules (primarily composed of chitin) preserved in lake sediments as a new proxy of past hydrological environmental changes. My developments have included: (1) assessing and modeling the potential of contamination sources during sample preparation; and (2) culturing chironomid larvae under controlled, replicated laboratory conditions, to examine the degree to which water and diet influence the delta18O and deltaD of chironomids. My growth experiment demonstrated that 69.0+/-0.4% of oxygen and 30.8+/-2.6% of hydrogen in chironomid larvae are derived from habitat water using a two-end member mixing model. The delta18O of chironomids remains can better constrain past habitat water isotopic changes compared to deltaD, due to 69% of the chironomid oxygen being influenced by habitat water. Having examined these methodological issues I then applied stable oxygen isotope analyses of fossil chironomid remains preserved in a sediment core from Idavain Lake, in southwest Alaska. The core represents the last ~16,000 years to the present and isotope analyses of chironomids from the core showed that the delta18O of past lake water had changed since deglaciation. Large variation in delta18O of chironomids (up to 20‰) are interpreted as alternating shifts in atmospheric flow regimes that are predominant in southwest Alaska, which are consistent with other evidence of past environmental changes at Idavain Lake (i.e. pollen, delta13C, delta15N, C/N). A zonal flow regime appears to have been dominant from 16,000 to 13,800 cal yr BP, 11,000 to 10,500 cal yr BP, 10,000 to 8,000 cal yr BP, and during a majority of the periods from 8,200 to 3,500 cal yr BP and from 2,000 cal yr BP to present. A mixed modern flow regime seems to have been dominant during the periods from 13,000 to 11,000 cal yr BP, 10,500 to 10,000 cal yr BP, 6,000 to 5,500 cal yr BP and 2,500 to 1,800 cal yr BP. These shifts in moisture regime appear to coincide with a series of glacier advances and recessions along the Gulf of Alaska. Thus, stable isotopic analysis of chironomid headcapsules is a promising tool for indicating paleoenvironmental change.