Accumulation and intense bioturbation of bioclastic muds along a carbonate-platform margin: Dry Tortugas, Florida

Bentley, Samuel J.; Nittrouer, Charles A.

doi:10.1016/j.margeo.2012.05.002

Cited by 7 publications

(4 citation statements)

References 54 publications

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“…This conceptual complexity translates into a three‐layer subdivision of sediments on the basis of depth and frequency of burrowing events (Berger et al, ), rather than the more conventional subdivision of sediments into a mixed and an unmixed layer. These three layers are the following: the surface completely‐mixed layer (SML), typically dominated by biodiffusion; the incompletely‐mixed layer (IML), inhabited by deeper infauna, whose lower boundary is effectively the maximum burrowing depth (referred to as the transition layer in analyses of bioturbation fabric; Bentley & Nittrouer, ; Ekdale et al, ; Löwemark, ; Savrda et al, ); and a layer with no active mixing, representing the zone of final burial (Olszewski, ). This stratification is sensitive to the timescale over which mixing is measured (Maire et al, ; Smith et al, ).…”

Section: Introductionmentioning

confidence: 99%

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

Tomášových

Kidwell

Alexander

et al. 2019

Paleoceanog and Paleoclimatol

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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 arẽ 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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Section: Introductionmentioning

confidence: 99%

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

Tomášových

Kidwell

Alexander

et al. 2019

Paleoceanog and Paleoclimatol

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“…However, this approach also has drawbacks because low sedimentation rates and bioturbational mixing typically generate time‐averaged assemblages (Anderson et al ., ; Scarponi & Kowalewski, ; Tomašových & Kidwell, ; Scarponi et al ., ) that can be inert to the most recent changes in ecosystem composition (Kidwell, , ). This inertia effect can be especially pronounced in carbonate environments with inherently low sedimentation rates and deep bioturbation (Kosnik et al ., , ; Bentley & Nittrouer, ). In this study, the effect of low sedimentation rate and bioturbational mixing on the temporal resolution of death assemblages is considered through dating a large number of shells (Tomašových & Kidwell, ; Tomašových et al ., ) and by using living assemblages as indicators of the most recent conditions unaffected by mixing (Kidwell, ; Tomašových et al ., ).…”

Section: Introductionmentioning

confidence: 99%

A decline in molluscan carbonate production driven by the loss of vegetated habitats encoded in the Holocene sedimentary record of the Gulf of Trieste

et al. 2018

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Carbonate sediments in non‐vegetated habitats on the north‐east Adriatic shelf are dominated by shells of molluscs. However, the rate of carbonate molluscan production prior to the 20th century eutrophication and overfishing on this and other shelves remains unknown because: (i) monitoring of ecosystems prior to the 20th century was scarce; and (ii) ecosystem history inferred from cores is masked by condensation and mixing. Here, based on geochronological dating of four bivalve species, carbonate production during the Holocene is assessed in the Gulf of Trieste, where algal and seagrass habitats underwent a major decline during the 20th century. Assemblages of sand‐dwelling Gouldia minima and opportunistic Corbula gibba are time‐averaged to >1000 years and Corbula gibba shells are older by >2000 years than shells of co‐occurring Gouldia minima . This age difference is driven by temporally disjunct production of two species coupled with decimetre‐scale mixing. Stratigraphic unmixing shows that Corbula gibba declined in abundance during the highstand phase and increased again during the 20th century. In contrast, one of the major contributors to carbonate sands – Gouldia minima – increased in abundance during the highstand phase, but declined to almost zero abundance over the past two centuries. Gouldia minima and herbivorous gastropods associated with macroalgae or seagrasses are abundant in the top‐core increments but are rarely alive. Although Gouldia minima is not limited to vegetated habitats, it is abundant in such habitats elsewhere in the Mediterranean Sea. This live–dead mismatch reflects the difference between highstand baseline communities (with soft‐bottom vegetated zones and hard‐bottom Arca beds) and present‐day oligophotic communities with organic‐loving species. Therefore, the decline in light penetration and the loss of vegetated habitats with high molluscan production traces back to the 19th century. More than 50% of the shells on the sea floor in the Gulf of Trieste reflect inactive production that was sourced by heterozoan carbonate factory in algal or seagrass habitats.

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“…Assuming relatively high sedimentation rates close to 1 cm/yr, that is, values close to the upper estimates of sedimentation rates on continental shelves (Sommerfield 2006), the twentieth-century sedimentary record will be captured in ~100-cm-long sediment cores. Such thickness is frequently affected by bioturbation reaching several decimeters or even a few meters below the sediment–water interface (Bentley and Nittrouer 2012; Parsons-Hubbard et al 2014). Stratigraphic patterns can be deconvolved to pre-bioturbation signals (Hull et al 2011; Steiner et al 2016) but such approaches require information on depth-dependent changes in bioturbation and can be limited under very slow sedimentation rate and high mixing (Schiffelbein 1985).…”

Section: Introductionmentioning

confidence: 99%

Tracing the effects of eutrophication on molluscan communities in sediment cores: outbreaks of an opportunistic species coincide with reduced bioturbation and high frequency of hypoxia in the Adriatic Sea

et al. 2018

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Estimating the effects and timing of anthropogenic impacts on the composition of macrobenthic communities is challenging, because early twentieth-century surveys are sparse and the corresponding intervals in sedimentary sequences are mixed by bioturbation. Here, to assess the effects of eutrophication on macrobenthic communities in the northern Adriatic Sea, we account for mixing with dating of the bivalve Corbula gibba at two stations with high accumulation (Po prodelta) and one station with moderate accumulation (Isonzo prodelta). We find that, first, pervasively bioturbated muds typical of highstand conditions deposited in the early twentieth century were replaced by muds with relicts of flood layers and high content of total organic carbon (TOC) deposited in the late twentieth century at the Po prodelta. The twentieth century shelly muds at the Isonzo prodelta are amalgamated but also show an upward increase in TOC. Second, dating of C. gibba shells shows that the shift from the early to the late twentieth century is characterized by a decrease in stratigraphic disorder and by an increase in temporal resolution of assemblages from ~25–50 years to ~10–20 years in both regions. This shift reflects a decline in the depth of the fully mixed layer from more than 20 cm to a few centimeters. Third, the increase in abundance of the opportunistic species C. gibba and the loss of formerly abundant, hypoxia-sensitive species coincided with the decline in bioturbation, higher preservation of organic matter, and higher frequency of seasonal hypoxia in both regions. This depositional and ecosystem regime shift occurred in ca. a.d. 1950. Therefore, the effects of enhanced food supply on macrobenthic communities were overwhelmed by oxygen depletion, even when hypoxic conditions were limited to few weeks per year in the northern Adriatic Sea. Preservation of trends in molluscan abundance and flood events in cores was enhanced by higher frequency of hypoxia that reduced bioturbation in the late twentieth century.

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Accumulation and intense bioturbation of bioclastic muds along a carbonate-platform margin: Dry Tortugas, Florida

Cited by 7 publications

References 54 publications

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

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

A decline in molluscan carbonate production driven by the loss of vegetated habitats encoded in the Holocene sedimentary record of the Gulf of Trieste

Tracing the effects of eutrophication on molluscan communities in sediment cores: outbreaks of an opportunistic species coincide with reduced bioturbation and high frequency of hypoxia in the Adriatic Sea

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