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Climate and ecosystem dynamics vary across timescales, but research into climate-driven vegetation dynamics usually focuses on singular timescales. We developed a spectral analysis–based approach that provides detailed estimates of the timescales at which vegetation tracks climate change, from 10¹to 10⁵years. We report dynamic similarity of vegetation and climate even at centennial frequencies (149⁻¹to 18,012⁻¹year⁻¹, that is, one cycle per 149 to 18,012 years). A breakpoint in vegetation turnover (797⁻¹year⁻¹) matches a breakpoint between stochastic and autocorrelated climate processes, suggesting that ecological dynamics are governed by climate across these frequencies. Heightened vegetation turnover at millennial frequencies (4650⁻¹year⁻¹) highlights the risk of abrupt responses to climate change, whereas vegetation-climate decoupling at frequencies >149⁻¹year⁻¹may indicate long-lasting consequences of anthropogenic climate change for ecosystem function and biodiversity. 

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. 

Abstract Understanding how time averaging changes during burial is essential for using Holocene and Anthropocene cores to analyze ecosystem change, given the many ways in which time averaging affects biodiversity measures. Here, we use transition-rate matrices to explore how the extent of time averaging changes downcore as shells transit through a taphonomically complex mixed layer into permanently buried historical layers: this is a null model, without any temporal changes in rates of sedimentation or bioturbation, to contrast with downcore patterns that might be produced by human activity. Assuming stochastic burial and exhumation movements of shellsbetweenincrements within the mixed layer and stochastic disintegrationwithinincrements, we find that almost all combinations of net sedimentation, mixing, and disintegration produce a downcore increase in time averaging (interquartile range [IQR] of shell ages), this trend is typically associated with a decrease in kurtosis and skewness and by a shift from right-skewed to symmetrical age distributions. A downcore increase in time averaging is thus the null expectation wherever bioturbation generates an internally structured mixed layer (i.e., a surface, well-mixed layer is underlain by an incompletely mixed layer): under these conditions, shells are mixed throughout the entire mixed layer at a slower rate than they are buried below it by sedimentation. This downcore trend created by mixing is further amplified by the downcore decline in disintegration rate. We find that transition-rate matrices accurately reproduce the downcore changes in IQR, skewness, and kurtosis observed in bivalve assemblages from the southern California shelf. The right-skewed shell age-frequency distributions typical of surface death assemblages—the focus of most actualistic research—might be fossilized under exceptional conditions of episodic anoxia or sudden burial. However, such right-skewed assemblages will typically not survive transit through the surface mixed layer into subsurface historical layers: they are geologically transient. The deep-time fossil record will be dominated instead by the more time-averaged assemblages with weakly skewed age distributions that form in the lower parts of the mixed layer. 

Abstract Near-time conservation palaeobiology uses palaeontological, archaeological and other geohistorical records to study the late Quaternary transition of the biosphere from its pristine past to its present-day, human-altered state. Given the scarcity of data on recent extinctions in the oceans, geohistorical records are critical for documenting human-driven extinctions and extinction threats in the marine realm. The historical perspective can provide two key insights. First, geohistorical records archive the state of pre-industrial oceans at local, regional and global scales, thus enabling the detection of recent extinctions and extirpations as well as shifts in species distribution, abundance, body size and ecosystem function. Second, we can untangle the contributions of natural and anthropogenic processes by documenting centennial-to-millennial changes in the composition and diversity of marine ecosystems before and after the onset of major human impacts. This long-term perspective identifies recently emerging patterns and processes that are unprecedented, thus allowing us to better assess human threats to marine biodiversity. Although global-scale extinctions are not well documented for brackish and marine invertebrates, geohistorical studies point to numerous extirpations, declines in ecosystem functions, increases in range fragmentation and dwindling abundance of previously widespread species, indicating that marine ecosystems are accumulating a human-driven extinction debt. 

Abstract Time averaging of fossil assemblages determines temporal precision of paleoecological and geochronological inferences. Taxonomic differences in intrinsic skeletal durability are expected to produce temporal mismatch between co-occurring species, but the importance of this effect is difficult to assess due to lack of direct estimates of time averaging for many higher taxa. Moreover, burial below the taphonomic active zone and early diagenetic processes may alleviate taxonomic differences in disintegration rates in subsurface sediments. We compared time averaging across five phyla of major carbonate producers co-occurring in a sediment core from the northern Adriatic Sea shelf. We dated individual bivalve shells, foraminiferal tests, tests and isolated plates of irregular and regular echinoids, crab claws, and fish otoliths. In spite of different skeletal architecture, mineralogy, and life habit, all taxa showed very similar time averaging varying from ~1800 to ~3600 yr (interquartile age ranges). Thus, remains of echinoids and crustaceans—two groups with multi-elemental skeletons assumed to have low preservation potential—can still undergo extensive age mixing comparable to that of the co-occurring mollusk shells. The median ages of taxa differed by as much as ~3700 yr, reflecting species-specific timing of seafloor colonization during the Holocene transgression. Our results are congruent with sequestration models invoking taphonomic processes that minimize durability differences among taxa. These processes together with temporal variability in skeletal production can overrule the effects of durability in determining temporal resolution of multi-taxic fossil assemblages. 

Abstract. Although the depth of bioturbation can be estimated on the basisof ichnofabric, the timescale of sediment mixing (reworking) and irrigation(ventilation) by burrowers that affects carbonate preservation andbiogeochemical cycles is difficult to estimate in the stratigraphic record.However, pyrite linings on the interior of shells can be a signature of slowand shallow irrigation. They indicate that shells of molluscs initiallyinhabiting oxic sediment pockets were immediately and permanentlysequestered in reduced, iron-rich microenvironments within the mixed layer.Molluscan biomass-stimulated sulfate reduction and pyrite precipitation wasconfined to the location of decay under such conditions. A high abundance ofpyrite-lined shells in the stratigraphic record can thus be diagnostic oflimited exposure of organic tissues to O2 even when the seafloor isinhabited by abundant infauna disrupting and age-homogenizing sedimentaryfabric as in the present-day northern Adriatic Sea. Here, we reconstructthis sequestration pathway characterized by slow irrigation (1) by assessingpreservation and postmortem ages of pyrite-lined shells of theshallow-infaunal and hypoxia-tolerant bivalve Varicorbula gibba in sediment cores and (2) byevaluating whether an independently documented decline in the depth ofmixing, driven by high frequency of seasonal hypoxia during the 20thcentury, affected the frequency of pyrite-lined shells in the stratigraphicrecord of the northern Adriatic Sea. First, at prodelta sites with a highsedimentation rate, linings of pyrite framboids form rapidly in the upper5–10 cm as they already appear in the interiors of shells younger than 10 yearsand occur preferentially in well-preserved and articulated shells withperiostracum. Second, increments deposited in the early 20th centurycontain < 20 % of shells lined with pyrite at the Po prodelta and30 %–40 % at the Isonzo prodelta, whereas the late 20th centuryincrements possess 50 %–80 % of shells lined with pyrite at both locations.At sites with slow sedimentation rate, the frequency of pyrite linings islow (< 10 %–20 %). Surface sediments remained well mixed by depositand detritus feeders even in the late 20th century, thus maintainingthe suboxic zone with dissolved iron. The upcore increase in the frequencyof pyrite-lined shells thus indicates that the oxycline depth was reducedand bioirrigation rates declined during the 20th century. Wehypothesize that the permanent preservation of pyrite linings within theshells of V. gibba in the subsurface stratigraphic record was enabled by slowrecovery of infaunal communities from seasonal hypoxic events, leading tothe dominance of surficial sediment modifiers with low irrigation potential.The presence of very young and well-preserved pyrite-lined valves in theuppermost zones of the mixed layer indicates that rapid obrution by episodicsediment deposition is not needed for preservation of pyrite linings whensediment irrigation is transient and background sedimentation rates arenot low (here, exceeding ∼ 0.1 cm yr−1) and infaunal organismsdie at their living position within the sediment. Abundance ofwell-preserved shells lined by pyrite exceeding ∼ 10 % perassemblage in apparently well-mixed sediments in the deep-time stratigraphicrecord can be an indicator of inefficient bioirrigation. Fine-grainedprodelta sediments in the northern Adriatic Sea deposited since themid-20th century, with high preservation potential of reducedmicroenvironments formed within a mixed layer, can represent taphonomic andearly diagenetic analogues of deep-time skeletal assemblages with pyritelinings. 

Abstract Studies of paleocommunities and trophic webs assume that multispecies assemblages consist of species that coexisted in the same habitat over the duration of time averaging. However, even species with similar durability can differ in age within a single fossil assemblage. Here, we tested whether skeletal remains of different phyla and trophic guilds, the most abundant infaunal bivalve shells and nektobenthic fish otoliths, differed in radiocarbon age in surficial sediments along a depth gradient from 10 to 40 m on the warm-temperate Israeli shelf, and we modeled their dynamics of taphonomic loss. We found that, in spite of the higher potential of fishes for out-of-habitat transport after death, differences in age structure within depths were smaller by almost an order of magnitude than differences between depths. Shell and otolith assemblages underwent depth-specific burial pathways independent of taxon identity, generating death assemblages with comparable time averaging, and supporting the assumption of temporal and spatial co-occurrence of mollusks and fishes.