ABSTRACT: Long-term records of river flow from isotopic variation in subfossil marine mollusks: The case of the Colorado River

Flessa, K.W.; Dettman, D.L.; Roopnarine, P.D.; Surge, D.M.; Goodfriend, G.A.; Zachos, J.C.; Tellez-Duarte, M.A. Geological Society of America Abstracts with Programs, vol. 29(6), A-66. (Salt Lake City, USA)

The oxygen isotope composition of subfossil bivalve mollusks from the Colorado Delta can be used to reconstruct the long-term variation in the discharge of the Colorado River. River discharge has been measured directly only for the past century. Estimates based on tree-ring variation are limited to the past 500 years and to the river's Upper Basin. How representative are these historic and proxy records for the past 1,000 yr of discharge at the river's mouth? Understanding the natural variation in the river's discharge is essential for planning allocations of river water during years of low flow.

We collect shells of the marine bivalve Chione fluctifraga from time-averaged, shelly shoals and cheniers from near the river's mouth. We use a radiocarbon-calibrated, amino acid (alloisoleucine/isoleucine) chronology to date the shells. Dating precision is comparable to AMS radiocarbon (+50yrs) and shells ranging in age back to 1000 AD are common.

Microsampling sections of dated shells allows the recovery of carbonate from individual growth bands. Analysis of live-collected shells shows that winters are easily recognized by translucent growth bands and positive excursions in d18O. Intra-shell variability d18O ranges from +0.06 to -2.2 per mil and is most likely the consequence of annual variation in water temperature because no fresh water now reaches the delta. A shell dated as ~1275 AD shows lower average d18O values and significantly higher intra-shell variability: values range from -0.03 to -5.8 per mil. We interpret the d18O variation in excess of that in the live-collected shell to be the result of the influx of river water to the delta. Significant negative excursions are most likely the result of high river discharge during the May-July arrival of snowmelt. Analyses of shells from each 50 year interval back to 1000 AD will provide a long term record of the river's annual flow and its seasonal variation. Shelly accumulations at the mouths of rivers can be used to provide a long term record of river flow.

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