An improved approach to age-modeling in deep time: Implications for the Santa Cruz Formation, Argentina

TRAYLER, ROBIN B.; SCHMITZ, MARK D.; CUITIÑO, JOSÉ I.; KOHN, MATTHEW J.; BARGO, M. SUSANA; KAY, RICHARD F.; STRÖMBERG, CAROLINE A.E.; VIZCAÍNO, SERGIO F.

GEOLOGICAL SOCIETY OF AMERICA BULLETIN

GEOLOGICAL SOC AMER, INC

Año: 2020

0016-7606

Accurate age-depth models for proxy recordsare crucial for inferring changes tothe environment through space and time,yet traditional methods of constructingthese models assume unrealistically smallage uncertainties and do not account formany geologic complexities. Here we modifyan existing Bayesian age-depth model to fosterits application for deep time U-Pb and40Ar/39Ar geochronology. More flexible inputlikelihood functions and use of an adaptiveproposal algorithm in the Markov ChainMonte Carlo engine better account for theage variability often observed in magmaticcrystal populations, whose dispersion canreflect inheritance, crystal residence timesand daughter isotope loss. We illustrate thisapproach by calculating an age-depth modelwith a contiguous and realistic uncertaintyenvelope for the Miocene Santa Cruz Formation(early Miocene; Burdigalian), Argentina.The model is calibrated using new,high-precision isotope dilution U-Pb zirconages for stratigraphically located interbeddedtuffs, whose weighted mean ages rangefrom ca. 16.78 ± 0.03 Ma to 17.62 ± 0.03 Ma.We document how the Bayesian age-depthmodel objectively reallocates probabilityacross the posterior ages of dated horizons,and thus produces better estimates of relativeages among strata and variations insedimentation rate. We also present a simplemethod to propagate age-depth model uncertaintiesonto stratigraphic proxy data usinga Monte Carlo technique. This approachallows us to estimate robust uncertainties onisotope composition through time, importantfor comparisons of terrestrial systemsto other proxy records.