21 Jun 2021

21 Jun 2021

Review status: this preprint is currently under review for the journal GChron.

New approaches to dating intermittently varved sediment, Columbine lake, Colorado, USA

Stephanie Harmonie Arcusa1, Nicholas P. McKay1, Charlotte Wiman2, Sela Patterson1, Samuel E. Munoz2,3, and Marco A. Aquino-López4 Stephanie Harmonie Arcusa et al.
  • 1School of Earth and Sustainability, Northern Arizona University, Flagstaff, 86011, USA
  • 2Department of Marine and Environmental Sciences, Northeastern University, Marine Science Center, Nahant, 01908, USA
  • 3Department of Civil and Environmental Engineering, Northeastern University, Boston, 02115, USA
  • 4Centro de Investigación en Matemáticas (CIMAT), Jalisco s/n, Valenciana, 36023 Guanajuato, Gto, Mexico

Abstract. Annually laminated lake sediment can track paleoenvironmental change at high-resolution where alternative archives are often not available. However, information about both paleoenvironmental change and chronology are often affected by indistinct and intermittent varves. We present an approach that overcomes these and other obstacles by using a quantitative varve quality index combined with a multi-core, multi-observer Bayesian varve sedimentation model that quantifies realistic under- and over-counting uncertainties while integrating information from radiometric measurements (210Pb, 137Cs, and 14C) into the chronology. We demonstrate this approach on thin sections of indistinct and intermittently varved sequences from alpine Columbine Lake, Colorado. The integrated model indicates 3137 (95 percentile highest density probability range: 2753–3375) varve years with a cumulative posterior distribution of counting uncertainties of −13/+7 % indicative of systematic observer undercounting. The sedimentary features of the thin and complex varves shift through time, from normally graded couplets to couplets interrupted with coarser sub-laminae, to inversely graded couplets. We interpret the normal grading couplets as spring nival discharge followed by winter settling, the coarser sub-laminae as high rainfall events, and the inverse grading as hyperpycnal flows and/or pulses of dust related to human impact changing the varve formation mechanism. Our novel approach provides a realistic constraint on sedimentation rates and quantifies uncertainty in varve counts by quantifying over- and under-counting uncertainties related to observer bias and the quality and variability of the sediment appearance. The approach permits the construction of a varve chronology and sedimentation rates for sites with intermittent or indistinct varves, which are likely more prevalent than sequences with distinct varves, and thus, expands the possibilities of reconstructing past environmental change with high resolution.

Stephanie Harmonie Arcusa et al.

Status: open (until 23 Aug 2021)

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Stephanie Harmonie Arcusa et al.

Model code and software

VarveR Nicholas P. McKay

Code for the varve and radiometric model integration Stephanie H. Arcusa, Nicholas P. McKay

Stephanie Harmonie Arcusa et al.


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Short summary
We present a 3100 year annually banded lake mud record, with an error of −13/+7 %, representing more undercounting than overcounting. Our results provide realistic sedimentation rates that can be used to calculate flux. Our method expands the possibility to overcome the obstacles of indistinct or interrupted banding that can affect the quality and amount of information about environmental change and age. Our results provide realistic sedimentation rates that can be used to calculate flux.