14 Lake Sedimentary Records of Past Climate and Environment

This session will focus on lacustrine archives and reconstructions of past environmental changes through the use of geochemical, biological, and sedimentological proxies. Both modern and paleo proxy datasets are welcome.

Sunday, 22 March 2026
1:30 PM - 5:30 PM
CCC, Room 27

If Your Age is too Old – a New Dating App

Paper number: 14-5
Program: Technical Sessions
Time: 2:55 PM – 3:15 PM

Authors

Abstract

Most 14C ages from lake Corg or carbonate precipitates are too old, labeled as ‘apparent ages’. Poor DIC equilibration with atmospheric 14C is one principal reason for these apparent ages. The point of ingress of 14CO2air into a water body is CO2aq, with rapid chemical and isotopic equilibration. Full DIC isotopic equilibration depends on the concentration and motility of the CO2aq species, and the HCO3- and CO3= concentrations. The equilibration rate between CO2aq and carbonate/bicarbonate equals tau = (DIC/CO2aq) (z/kw), where tau is the e-folding time of equilibration, z thickness of the mixed layer, and kw is the piston velocity of dissolved CO2 gas. The first ratio is pH dependent, and z/Kw depends largely on physical lake processes. The air-water 14C equilibration can be counteracted by lake inflows of 14C-poor carbonate-rich fluids, and then the DIC concentration becomes the dominant rate factor. We use the mismatch between real and apparent ages as a tool to retrodict either lake water pH (dilute lakes with minor/no inputs of carbonate-rich liquids) or approximate paleo-DIC for lakes with substantial 14C-free carbonate inputs. The 14C dating equation is t = -8033 ln Fm, where Fm equals Nt/No and t is the 14C age; Nt is the remaining 14C in the sample after t years, and No is the original 14C contents during plant growth or carbonate precipitation, traditionally taken as the 1950 14C standard value. The difference between apparent and real ages at time tx in the past is normalized on the same age difference in modern (tm) lake DIC. We introduce a new variable No*, which represents No corrected for the poor isotopic 14C equilibration of the lake water.  After a lengthy derivation, the obtained master equation xNo*/mNo*= e-DDt/8033 relates the difference between the differences between apparent and real ages at times tx and tm to the ratio of the No* values at these two times. We re-phrase the No* factors in terms of No and tau, the latter linked to water pH. We estimate the paleo-DIC if fluid mixing is the dominant process for the 14C air-water disequilibrium by converting No into No* with a 1/DIC scaling parameter.  We apply this approach to 14C data from two volcanic lakes at Newberry Volcano (OR).

Geological Society of America Abstracts with Programs. Vol. 58, No. 2

doi: 10.1130/abs/2026NE-411304

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