TY - JOUR
T1 - Millennial-Scale Age Offsets Within Fossil Assemblages
T2 - Result of Bioturbation Below the Taphonomic Active Zone and Out-of-Phase Production
AU - Tomašových, A.
AU - Kidwell, S. M.
AU - Alexander, C. R.
AU - Kaufman, D. S.
N1 - Funding Information:
All data are deposited at doi.pangaea.de/10.1594/PANGAEA.900879. Cores were acquired during cruise MV1211 of the RV Melville out of the University of California San Diego. For field assistance we thank the crew; undergraduate and graduate student volunteers from the University of Chicago, Savannah State University, and UCSD; and professional volunteers T. Parker and C. MacDonald (LACSD), B. Edwards (USGS), R. Cipriani (Santa Monica), and J. McNinch and H. Wadman (USACE), who provided crucial vibracore equipment. For field and lab assistance we thank L. DeLeo, M. Robinson, and C. Venherm (Skidaway) and J. Leonard-Pingel (Ohio State University); and for lab assistance Katherine Whitacre (NAU). For access to historical data on living bivalves of the Palos Verdes and San Pedro shelves and biological insights, we are grateful to the Sanitation Districts of Los Angeles County and the Orange County Sanitation District, particularly D. Cadien, W. Powers, and K. Barwick for facilitating approval. We thank two reviewers for helpful reviews of the manuscript. Research supported by NSF EAR-112418 (S. K. and A. T.) and 1124318 (CA), with pilot work supported by a National Oceanic and Atmospheric Association SeaGrant administered by the University of Southern California (NA07OAR4170008), by the Slovak Research and Development Agency (APVV17-0555), and the Slovak Scientific Graint Agency (VEGA 0169/19).
Funding Information:
All data are deposited at doi.pangaea. de/10.1594/PANGAEA.900879. Cores were acquired during cruise MV1211 of the RV Melville out of the University of California San Diego. For field assistance we thank the crew; undergraduate and graduate student volunteers from the University of Chicago, Savannah State University, and UCSD; and professional volunteers T. Parker and C. MacDonald (LACSD), B. Edwards (USGS), R. Cipriani (Santa Monica), and J. McNinch and H. Wadman (USACE), who provided crucial vibracore equipment. For field and lab assistance we thank L. DeLeo, M. Robinson, and C. Venherm (Skidaway) and J. Leonard‐Pingel (Ohio State University); and for lab assistance Katherine Whitacre (NAU). For access to historical data on living bivalves of the Palos Verdes and San Pedro shelves and biological insights, we are grateful to the Sanitation Districts of Los Angeles County and the Orange County Sanitation District, particularly D. Cadien, W. Powers, and K. Barwick for facilitating approval. We thank two reviewers for helpful reviews of the manuscript. Research supported by NSF EAR‐112418 (S. K. and A. T.) and 1124318 (CA), with pilot work supported by a National Oceanic and Atmospheric Association SeaGrant administered by the University of Southern California (NA07OAR417 0008), by the Slovak Research and Development Agency (APVV17‐0555), and the Slovak Scientific Graint Agency (VEGA 0169/19).
Publisher Copyright:
© 2019. American Geophysical Union. All Rights Reserved.
PY - 2019/6
Y1 - 2019/6
N2 - 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.
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.
KW - California
KW - bioturbation
KW - geochronology
KW - sedimentation rate
KW - surface mixed layer
KW - taphonomic active zone
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U2 - 10.1029/2018PA003553
DO - 10.1029/2018PA003553
M3 - Article
AN - SCOPUS:85068183973
SN - 2572-4517
VL - 34
SP - 954
EP - 977
JO - Paleoceanography and Paleoclimatology
JF - Paleoceanography and Paleoclimatology
IS - 6
ER -