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Abstract Ice-sheet volume during Marine Isotope Stage (MIS) 3 (57–29 ka) is controversial. Several recent studies have proposed that the Greenland Ice Sheet was smaller during MIS 3 than it is today based on radiocarbon ages of molluscan bivalve shells reworked into sedimentary deposits adjacent to the present ice margin. Such a result contrasts with available records of MIS 3 climate, ice volume, and sea level. We revisited a site previously interpreted as containing evidence for smaller than present ice during MIS 3. We collected marine bivalve shells and combined progressive acid dissolution in preparation for radiocarbon dating with new-generation amino acid analysis, which focuses on aspartic acid racemization. Our results suggest that contamination by young carbon yields finite radiocarbon ages despite bivalve shells likely dating to MIS 5e (∼125 ka) or even older. This result should be further tested, which could be accomplished with additional studies of this kind in combination with ice-sheet modeling and additional paleoclimate data generated from adjacent seas. 

ABSTRACT Bioerosion is a valuable tool for inferring palaeoenvironmental and palaeoclimatic changes over time and across different regions. However, studies of bioerosion traces are scarce in the Southern Hemisphere. Most ichnological studies within Argentina are concentrated in San Jorge Gulf (Patagonia, Argentina) and little is known about deposits located north of the Gulf. Here, we focus on bioerosion traces on Quaternary mollusc shells. Samples were collected from Quaternary marine deposits at the Bahía Vera–Cabo Raso sites in northern San Jorge Gulf. To resolve age discrepancies reported in the literature, we use amino acid racemization and radiocarbon dating to confirm the presence of beach ridge deposits from Marine Isotope Stage (MIS) 5 and MIS 1. Fourteen ichnotaxa are recorded in the study area. Additionally, distinct variations in the pattern of bioerosion across different ages are observed, indicating that environmental changes occurred in the northern San Jorge Gulf between the MIS 5 interglacial and the Holocene. This reinforces the hypothesis that there is an association between bioerosion, productivity and circulation in the Southern Atlantic Ocean. 

Abstract Bioturbation can increase time averaging by downward and upward movements of young and old shells within the entire mixed layer and by accelerating the burial of shells into a sequestration zone (SZ), allowing them to bypass the uppermost taphonomically active zone (TAZ). However, bioturbation can increase shell disintegration concurrently, neutralizing the positive effects of mixing on time averaging. Bioirrigation by oxygenated pore-water promotes carbonate dissolution in the TAZ, and biomixing itself can mill shells weakened by dissolution or microbial maceration, and/or expose them to damage at the sediment–water interface. Here, we fit transition rate matrices to bivalve age–frequency distributions from four sediment cores from the southern California middle shelf (50–75 m) to assess the competing effects of bioturbation on disintegration and time averaging, exploiting a strong gradient in rates of sediment accumulation and bioturbation created by historic wastewater pollution. We find that disintegration covaries positively with mixing at all four sites, in accord with the scenario where bioturbation ultimately fuels carbonate disintegration. Both mixing and disintegration rates decline abruptly at the base of the 20- to 40-cm-thick, age-homogenized surface mixed layer at the three well-bioturbated sites, despite different rates of sediment accumulation. In contrast, mixing and disintegration rates are very low in the upper 25 cm at an effluent site with legacy sediment toxicity, despite recolonization by bioirrigating lucinid bivalves. Assemblages that formed during maximum wastewater emissions vary strongly in time averaging, with millennial scales at the low-sediment accumulation non-effluent sites, a centennial scale at the effluent site where sediment accumulation was high but bioturbation recovered quickly, and a decadal scale at the second high-sedimentation effluent site where bioturbation remained low for decades. Thus, even though disintegration rates covary positively with mixing rates, reducing postmortem shell survival, bioturbation has theneteffect of increasing the time averaging of skeletal remains on this warm-temperate siliciclastic shelf.