354-6 A North American perspective of the mercury cycle across the Cretaceous-Paleogene transition

Abstract:

The North American Interior preserves spectacular terrestrial deposits that span the Cretaceous-Paleogene (KPg) boundary and the end-Cretaceous mass extinction (ECME). Many sections also include evidence for the recovery of ecosystems in the aftermath of the ECME over the first million years of the Paleogene. Although geological evidence points to an extraterrestrial impact causing the ECME, the emplacement history of Deccan volcanism during the Danian (early Paleogene) may represent an important factor controlling climatic and environmental change, and therefore the rebuilding of ecosystems on land. Investigating the record of sedimentary mercury (Hg) has the capacity to resolve the timing of the impact and volcanic activity as well as biogeochemical feedbacks to these phenomena including changes in soil formation, wildfires, and rainfall patterns.

We present a latitudinal transect of high-resolution (cm- to dm-scale) sedimentary Hg and total organic carbon (TOC) concentrations, as well as, fossil and other geochemical data including U-Pb radiometric age dates of ash layers, across the study interval. This drill core- and outcrop-based transect, spans North Dakota to New Mexico, allowing us to reconstruct regional Hg cycling and remove any bias that singular sites have. There are widespread Hg and Hg/TOC enrichments surrounding the KPg boundary, but diagenetic alteration appears to have erased these enrichments at one site. In sites without a clear KPg boundary clay, Hg and Hg/TOC enrichments may or may not be present, which is likely controlled by sediment accumulation and preservation during this transient interval. We find clear evidence that secondary mineral formation after deposition may also impact the Hg and Hg/TOC records. New Hg isotope (Δ¹⁹⁹Hg and δ²⁰²Hg) data from a section that preserves the highest resolution KPg boundary interval also supports a rapid drawdown atmospheric Hg in the aftermath of the impact, but is not what would be expected if Hg was sourced from Deccan volcanism. Based on this compilation, we find it unlikely that KPg terrestrial Hg records can be used to estimate Hg fluxes from Deccan volcanism due to the influence of local processes that lead to unique Hg geochemical records. More broadly, this research confirms and highlights that investigating multiple sites representing different depositional environments is required to properly apply the Hg geochemical proxy to sediments and sedimentary rock.

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