Millennial-Scale Age Offsets Within Fossil Assemblages: Result of Bioturbation Below the Taphonomic Active Zone and Out-of-Phase Production

A. Tomašových, S. M. Kidwell, C. R. Alexander, Darrell S Kaufman

Research output: Contribution to journalArticle

Abstract

Oceanographic and evolutionary inferences based on fossil assemblages can be obscured by age offsets among co-occurring shells (i.e., time averaging). To identify the contributions of sedimentation, mixing, durability, and production to within- and between-species age offsets, we analyze downcore changes in the age-frequency distributions of two bivalves on the California shelf. Within-species age offsets are ~50–2,000 years for Parvilucina and ~2,000–4,000 years for Nuculana and between-species offsets are 1,000–4,000 years within the 10- to 25-cm-thick stratigraphic units. Shells within the top 20–24 cm of the seabed are age-homogeneous, defining the thickness of the surface completely-mixed layer (SML), and have strongly right-skewed age-frequency distributions, indicating fast shell disintegration. The SML thus coincides with the taphonomic active zone and extends below the redoxcline at ~10 cm. Shells >2,000–3,000 years old occurring within the SML have been exhumed from subsurface shell-rich units rich where disintegration is negligible (sequestration zone, SZ). Burrowers (callianassid shrimps) penetrate 40–50 cm below the seafloor into this SZ. The millennial offsets within each increment result from the advection of old shells from the SZ, combined with an out-of-phase change in species production. Age unmixing reveals that Parvilucina was abundant during the transgressive phase, rare during the highstand phase, and increased steeply in the twentieth century in response to wastewater. Nuculana was abundant during the highstand phase and has declined over the past two centuries. This sequestration-exhumation dynamic accentuates age offsets by allowing both the persistence of shells below the SML and their later admixing with younger shells within the SML.

Original languageEnglish (US)
Pages (from-to)954-977
Number of pages24
JournalPaleoceanography and Paleoclimatology
Volume34
Issue number6
DOIs
StatePublished - Jun 1 2019

Fingerprint

fossil assemblage
bioturbation
shell
mixed layer
surface layer
highstand
durability
exhumation
twentieth century
bivalve
advection
seafloor
persistence
sedimentation
wastewater

Keywords

  • bioturbation
  • California
  • geochronology
  • sedimentation rate
  • surface mixed layer
  • taphonomic active zone

ASJC Scopus subject areas

  • Oceanography
  • Atmospheric Science
  • Palaeontology

Cite this

Millennial-Scale Age Offsets Within Fossil Assemblages : Result of Bioturbation Below the Taphonomic Active Zone and Out-of-Phase Production. / Tomašových, A.; Kidwell, S. M.; Alexander, C. R.; Kaufman, Darrell S.

In: Paleoceanography and Paleoclimatology, Vol. 34, No. 6, 01.06.2019, p. 954-977.

Research output: Contribution to journalArticle

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AB - Oceanographic and evolutionary inferences based on fossil assemblages can be obscured by age offsets among co-occurring shells (i.e., time averaging). To identify the contributions of sedimentation, mixing, durability, and production to within- and between-species age offsets, we analyze downcore changes in the age-frequency distributions of two bivalves on the California shelf. Within-species age offsets are ~50–2,000 years for Parvilucina and ~2,000–4,000 years for Nuculana and between-species offsets are 1,000–4,000 years within the 10- to 25-cm-thick stratigraphic units. Shells within the top 20–24 cm of the seabed are age-homogeneous, defining the thickness of the surface completely-mixed layer (SML), and have strongly right-skewed age-frequency distributions, indicating fast shell disintegration. The SML thus coincides with the taphonomic active zone and extends below the redoxcline at ~10 cm. Shells >2,000–3,000 years old occurring within the SML have been exhumed from subsurface shell-rich units rich where disintegration is negligible (sequestration zone, SZ). Burrowers (callianassid shrimps) penetrate 40–50 cm below the seafloor into this SZ. The millennial offsets within each increment result from the advection of old shells from the SZ, combined with an out-of-phase change in species production. Age unmixing reveals that Parvilucina was abundant during the transgressive phase, rare during the highstand phase, and increased steeply in the twentieth century in response to wastewater. Nuculana was abundant during the highstand phase and has declined over the past two centuries. This sequestration-exhumation dynamic accentuates age offsets by allowing both the persistence of shells below the SML and their later admixing with younger shells within the SML.

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