scholarly journals Re-evaluating <sup>14</sup>C dating accuracy in deep-sea sediment archives

2019 ◽  
Author(s):  
Bryan C. Lougheed ◽  
Philippa Ascough ◽  
Andrew M. Dolman ◽  
Ludvig Löwemark ◽  
Brett Metcalfe
Keyword(s):  
Deep Sea ◽  
Large Scale ◽  
Global Climate ◽  
Age Distribution ◽  
Sediment Accumulation ◽  
Depth Interval ◽  
14C Dating ◽  
Deep Sea Sediment ◽  
Calibration Process ◽  
14C Age

Abstract. The current geochronological state-of-the-art for applying the radiocarbon (14C) method to deep-sea sediment archives lacks key information on sediment bioturbation. Here, we apply a sediment accumulation model that simulates the sedimentation and bioturbation of millions of foraminifera, whereby realistic 14C activities (i.e. from a 14C calibration curve) are assigned to each single foraminifera based on its simulation timestep. We find that the normal distribution of 14C age typically used to represent discrete-depth sediment intervals (based on the reported laboratory 14C age and measurement error) is unlikely to be a faithful reflection of the actual 14C age distribution for a specific depth interval. We also find that this deviation from the actual 14C age distribution is greatly amplified during the calibration process. We find a systematic underestimation of total geochronological error in many cases (by up to thousands of years), as well as the generation of age-depth artefacts in downcore calibrated median age. Specifically, we find that even in the case of perfect simulated sediment archive scenarios, whereby sediment accumulation rate (SAR), bioturbation depth, reservoir age and species abundance are all kept constant, the 14C dating and calibration process generates temporally dynamic median age-depth artefacts, on the order of hundreds of years – even in the case of high SAR scenarios of 40 cm ka−1 and 60 cm ka−1. Such age-depth artefacts can be especially pronounced during periods corresponding to dynamic changes in the Earth's Δ14C, where single foraminifera of varying 14C activity can be incorporated into single discrete-depth sediment intervals. In certain SAR scenarios, a discrete depth’s true median age can consistently fall outside the 95.45 % calibrated age range predicted by the 14C dating and calibration process. Our findings suggest the possibility of 14C-derived age-depth artefacts in the literature: since age-depth artefacts are likely to coincide with large-scale changes in global Δ14C, which themselves can coincide with large-scale changes in global climate (such as the last deglaciation), 14C-derived age-depth artefacts may have been previously been (partially) misinterpreted as due to changes in global climate. Our study highlights the need for the development of improved deep-sea sediment 14C calibration techniques that include an a priori representation of bioturbation for multi-specimen samples.

scholarly journals Re-evaluating <sup>14</sup>C dating accuracy in deep-sea sediment archives

Geochronology ◽  
2020 ◽  
Vol 2 (1) ◽  
pp. 17-31 ◽  
Author(s):  
Bryan C. Lougheed ◽  
Philippa Ascough ◽  
Andrew M. Dolman ◽  
Ludvig Löwemark ◽  
Brett Metcalfe
Keyword(s):  
Deep Sea ◽  
Large Scale ◽  
Accumulation Rate ◽  
Global Climate ◽  
Age Distribution ◽  
Sediment Accumulation ◽  
Depth Interval ◽  
Time Step ◽  
Deep Sea Sediment ◽  
14C Age

Abstract. The current geochronological state of the art for applying the radiocarbon (14C) method to deep-sea sediment archives lacks key information on sediment bioturbation. Here, we apply a sediment accumulation model that simulates the sedimentation and bioturbation of millions of foraminifera, whereby realistic 14C activities (i.e. from a 14C calibration curve) are assigned to each single foraminifera based on its simulation time step. We find that the normal distribution of 14C age typically used to represent discrete-depth sediment intervals (based on the reported laboratory 14C age and measurement error) is unlikely to be a faithful reflection of the actual 14C age distribution for a specific depth interval. We also find that this deviation from the actual 14C age distribution is greatly amplified during the calibration process. Specifically, we find a systematic underestimation of total geochronological error in many cases (by up to thousands of years), as well as the generation of age–depth artefacts in downcore calibrated median age. Even in the case of “perfect” simulated sediment archive scenarios, whereby sediment accumulation rate (SAR), bioturbation depth, reservoir age and species abundance are all kept constant, the 14C measurement and calibration processes generate temporally dynamic median age–depth artefacts on the order of hundreds of years – whereby even high SAR scenarios (40 and 60 cm kyr−1) are susceptible. Such age–depth artefacts can be especially pronounced during periods corresponding to dynamic changes in the Earth's Δ14C history, when single foraminifera of varying 14C activity can be incorporated into single discrete-depth sediment intervals. For certain lower-SAR scenarios, we find that downcore discrete-depth true median age can systematically fall outside the calibrated age range predicted by the 14C measurement and calibration processes, thus leading to systematically inaccurate age estimations. In short, our findings suggest the possibility of 14C-derived age–depth artefacts in the literature. Furthermore, since such age–depth artefacts are likely to coincide with large-scale changes in global Δ14C, which themselves can coincide with large-scale changes in global climate (such as the last deglaciation), 14C-derived age–depth artefacts may have been previously incorrectly attributed to changes in SAR coinciding with global climate. Our study highlights the need for the development of improved deep-sea sediment 14C calibration techniques that include an a priori representation of bioturbation for multi-specimen samples.

scholarly journals Using sedimentological priors to improve 14C calibration of bioturbated sediment archives.

2021 ◽  
Author(s):  
Bryan Lougheed
Keyword(s):  
Deep Sea ◽  
Accumulation Rate ◽  
Age Distribution ◽  
Sediment Accumulation ◽  
Species Abundance ◽  
Homogeneous Material ◽  
14C Dating ◽  
Deep Sea Sediment ◽  
Lack Of Information ◽  
Calibration Protocol

Radiocarbon (14C) dating is often carried out upon multi-specimen samples sourced from bioturbated sediment archives, such as deep-sea sediment. These samples are inherently heterogeneous in age, but current 14C calibration techniques applied to such age heterogenous samples were originally developed for age homogeneous material. A lack of information about age heterogeneity leads to a systematic underestimation of a sample's true age range, as well as the possible generation of significant age-depth artefacts during periods of highly dynamic Δ14C. Here, a calibration protocol is described that allows for the application of sedimentological priors describing sediment accumulation rate, bioturbation depth and temporally dynamic species abundance. This Bayesian approach produces a credible calibrated age distribution associated with a particular laboratory 14C determination and its associated sedimentological priors, resulting in an improved calibration, especially in the case of low sediment accumulation rates typical of deep-sea sediment. A time-optimised computer script (biocal) for the new calibration protocol is also presented, thus allowing for rapid and automated application of the new calibration protocol using sedimentological priors. This new calibration protocol can be applied within existing age-depth modelling software packages to produce more accurate geochronologies for bioturbated sediment archives.

scholarly journals SEAMUS (v1.0): a Δ<sup>14</sup>C-enabled, single-specimen sediment accumulation simulator

2019 ◽  
Author(s):  
Bryan C. Lougheed
Keyword(s):  
Sediment Core ◽  
Accumulation Rate ◽  
Sediment Accumulation ◽  
Species Abundance ◽  
Depth Interval ◽  
Single Specimen ◽  
14C Dating ◽  
Deep Sea Sediment ◽  
Wide Range ◽  
History Of

Abstract. The systematic bioturbation of single particles (such as foraminifera) within deep-sea sediment archives leads to the apparent smoothing of any temporal signal as record by the downcore, discrete-depth mean signal. This smoothing is the result of the systematic mixing of particles from a wide range of depositional ages into the same discrete depth interval. Previous sediment models that simulate bioturbation have specifically produced an output in the form of a downcore, discrete-depth mean signal. Palaeoceanographers analysing the distribution of single foraminifera specimens from sediment core intervals would be assisted by a model that specifically evaluates the effect of bioturbation upon single specimen populations. Taking advantage of recent increases in computer memory, the single-specimen SEdiment AccuMUlation Simulator (SEAMUS) was created in Matlab, whereby large arrays of single specimens are simulated. This simulation allows researchers to analyse the post-bioturbation age heterogeneity of single specimens contained within discrete-depth sediment core intervals, and how this heterogeneity is influenced by changes in sediment accumulation rate (SAR), bioturbation depth (BD) and species abundance. The simulation also assigns a realistic 14C activity to each specimen, by considering the dynamic Δ14C history of the Earth and temporal changes in reservoir age. This approach allows for the quantification of possible significant artefacts arising when 14C dating multi-specimen samples with heterogeneous 14C activity. Users may also assign additional desired carrier signals to specimens (e.g., stable isotopes, trace elements, temperature, etc.) and consider a second species with an independent abundance. Finally, the model can simulate a virtual palaeoceanographer by randomly picking whole specimens (whereby the user can set the percentage of older, broken specimens) of a prescribed sample size from discrete depths, after which virtual laboratory 14C dating and 14C calibration is carried out within the model.

scholarly journals Moving beyond the age–depth model paradigm in deep-sea palaeoclimate archives: dual radiocarbon and stable isotope analysis on single foraminifera

2018 ◽  
Vol 14 (4) ◽  
pp. 515-526 ◽  
Author(s):  
Bryan C. Lougheed ◽  
Brett Metcalfe ◽  
Ulysses S. Ninnemann ◽  
Lukas Wacker
Keyword(s):  
Stable Isotope ◽  
Deep Sea ◽  
Accumulation Rate ◽  
Isotope Analysis ◽  
Sediment Accumulation ◽  
Late Glacial ◽  
Valuable Insight ◽  
14C Dating ◽  
Deep Sea Sediment ◽  
Ocean Chemistry

Abstract. Late-glacial palaeoclimate reconstructions from deep-sea sediment archives provide valuable insight into past rapid changes in ocean chemistry. Unfortunately, only a small proportion of the ocean floor with sufficiently high sediment accumulation rate (SAR) is suitable for such reconstructions using the long-standing age–depth model approach. We employ ultra-small radiocarbon (14C) dating on single microscopic foraminifera to demonstrate that the long-standing age–depth model method conceals large age uncertainties caused by post-depositional sediment mixing, meaning that existing studies may underestimate total geochronological error. We find that the age–depth distribution of our 14C-dated single foraminifera is in good agreement with existing bioturbation models only after one takes the possibility of Zoophycos burrowing into account. To overcome the problems associated with the age–depth paradigm, we use the first ever dual 14C and stable isotope (δ18O and δ13C) analysis on single microscopic foraminifera to produce a palaeoclimate time series independent of the age–depth paradigm. This new state of the art essentially decouples single foraminifera from the age–depth paradigm to provide multiple floating, temporal snapshots of ocean chemistry, thus allowing for the successful extraction of temporally accurate palaeoclimate data from low-SAR deep-sea archives. This new method can address large geographical gaps in late-glacial benthic palaeoceanographic reconstructions by opening up vast areas of previously disregarded, low-SAR deep-sea archives to research, which will lead to an improved understanding of the global interaction between oceans and climate.

scholarly journals SEAMUS (v1.20): a Δ<sup>14</sup>C-enabled, single-specimen sediment accumulation simulator

2020 ◽  
Vol 13 (1) ◽  
pp. 155-168 ◽  
Author(s):  
Bryan C. Lougheed
Keyword(s):  
Sediment Core ◽  
Accumulation Rate ◽  
Sediment Accumulation ◽  
Species Abundance ◽  
Depth Interval ◽  
Single Specimen ◽  
14C Dating ◽  
Total Uncertainty ◽  
Deep Sea Sediment ◽  
Wide Range

Abstract. The systematic bioturbation of single particles (such as foraminifera) within deep-sea sediment archives leads to the apparent smoothing of any temporal signal as recorded by the downcore, discrete-depth mean signal. This smoothing is the result of the systematic mixing of particles from a wide range of depositional ages into the same discrete-depth interval. Previous sediment models that simulate bioturbation have specifically produced an output in the form of a downcore, discrete-depth mean signal. However, palaeoceanographers analysing the distribution of single foraminifera specimens from sediment core intervals would be assisted by a model that specifically evaluates the effect of bioturbation upon single specimens. Taking advantage of advances in computer memory, the single-specimen SEdiment AccuMUlation Simulator (SEAMUS) was created for MATLAB and Octave, allowing for the simulation of large arrays of single specimens. This model allows researchers to analyse the post-bioturbation age heterogeneity of single specimens contained within discrete-depth sediment core intervals and how this heterogeneity is influenced by changes in sediment accumulation rate (SAR), bioturbation depth (BD) and species abundance. The simulation also assigns a realistic 14C activity to each specimen, by considering the dynamic Δ14C history of the Earth and temporal changes in reservoir age. This approach allows for the quantification of possible significant artefacts arising when 14C-dating multi-specimen samples with heterogeneous 14C activity. Users may also assign additional desired carrier signals to single specimens (stable isotopes, trace elements, temperature, etc.) and consider a second species with an independent abundance. Finally, the model can simulate a virtual palaeoceanographer by randomly picking whole specimens (whereby the user can set the percentage of older, “broken” specimens) of a prescribed sample size from discrete depths, after which virtual laboratory 14C dating and 14C calibration is carried out within the model. The SEAMUS bioturbation model can ultimately be combined with other models (proxy and ecological models) to produce a full climate-to-sediment model workflow, thus shedding light on the total uncertainty involved in palaeoclimate reconstructions based on sediment archives.

scholarly journals Re-evaluating 14C dating accuracy in deep-sea sediment archives.

2019 ◽  
Author(s):  
Bryan Lougheed ◽  
Philippa Ascough ◽  
Andrew Dolman ◽  
Ludvig Löwemark ◽  
Brett Metcalfe
Keyword(s):  
Deep Sea ◽  
14C Dating ◽  
Deep Sea Sediment

Global wind-induced change of deep-sea sediment budgets, new ocean production and CO 2 reservoirs ca . 3.3-2.35 Ma BP

1988 ◽  
Vol 318 (1191) ◽  
pp. 487-504 ◽  
Keyword(s):  
Large Scale ◽  
Accumulation Rate ◽  
Sediment Accumulation ◽  
Distribution Patterns ◽  
Trade Wind ◽  
Accumulation Rates ◽  
Deep Sea Sediment ◽  
Late Pliocene ◽  
High Productivity ◽  
Sediment Fraction

The late Pliocene phase of large-scale climatic deterioration about 3.2-2.4 Ma BP is well documented in a number of (benthic) δ 18 O records. To test the global implications of this event, we have mapped the distribution patterns of various sediment variables in the Pacific and Atlantic Oceans during two time slices, 3.4-3.18 and 2.43-2.33 Ma BP. The changes of bulk sedimentation and bulk sediment accumulation rates are largely explained by the variations of CaCO 3 -accumulation rates (and the accumulation rates of the complementary siliciclastic sediment fraction near continents in higher latitudes). During the late Pliocene, the CaCO 3 -accumulation rate increased along the equatorial Pacific and Atlantic and in the northeastern Atlantic, but decreased elsewhere. The accumulation rate of organic carbon (C org ) and net palaeoproductivity also increased below the high-productivity belts along the equator and the eastern continental margins. From these patterns we may conclude that (trade-) wind- induced upwelling zones and upwelling productivity were much enhanced during that time. This change led to an increased transfer of CO 2 from the surface ocean to the ocean deep water and to a reduction of evaporation, which resulted in an aridification of the Saharan desert belt as depicted in the dust sediments off northwest Africa.

Matching Mind and Method with Material: John Imbrie and Quantitative Facies Analysis

2011 ◽  
Vol 30 (1) ◽  
pp. 163-171 ◽  
Author(s):  
Léo Laporte
Keyword(s):  
Deep Sea ◽  
Large Scale ◽  
Facies Analysis ◽  
Global Climate ◽  
Sedimentary Facies ◽  
Major Axis ◽  
Growth Patterns ◽  
Lower Permian ◽  
Milankovitch Cycles ◽  
Scientific Success

John Imbrie (b. 1925) always had deep mathematical insight and facility. At Yale University he completed his PhD (1951) under Carl Dunbar working on Middle Devonian brachiopods where he employed a statistical technique—'reduced major axis regression'—to differentiate several subspecies. Later, in a study with Edwin Colbert at the American Museum of Natural History, he used the same technique to determine subtle, yet significant, variations in the growth patterns of Triassic Metoposaurid amphibians (1956). At about the same time as sedimentary facies analysis was becoming of increased interest, Imbrie sought to test what one might do with quantitative facies analysis by undertaking a decade-long study of the Lower Permian Florena Shale (Kansas) using multivariate cluster analysis to characterize different litho- and biofacies. Despite much hard work in the field and with a highdecibel desk calculator, the hoped for results were lackluster. But neither the man nor the methods were wanting. The materials—fragmented, scattered invertebrate fossils imbedded in shales and limestones—permitted no more understanding than qualitative, eye-ball analysis. Even a late stage attack with the IBM computer at Columbia University merely groaned and brought forth similar mousey results. What was needed was a problem whose material components (abundant planktonic microfossils) within well-characterized stratigraphic sequences (deep-sea Pleistocene cores) were suitably matched to the man's mind and his quantitative procedures. And, of course, the result was phenomenal: his empirical demonstration of the deep-sea data for the validity of Milankovitch Cycles as the forcing factors for large-scale global climate change. His scientific success was duly honored by awards, prizes, medals, and elections to distinguished honorary societies. How did this happen?

scholarly journals Species distribution modeling of deep sea sponges in the North Pacific Ocean.

2018 ◽  
Author(s):  
Fiona Davidson
Keyword(s):  
Predictive Modeling ◽  
North Pacific ◽  
Deep Sea ◽  
Large Scale ◽  
Global Climate ◽  
North Pacific Ocean ◽  
The North ◽  
Early Results ◽  
Presence Data ◽  
The North Pacific

Knowledge of deep-sea species and their ecosystems is limited due to the inaccessibility of the areas and the prohibitive cost of conducting large-scale field studies. My graduate research has used predictive modeling methods to map hexactinellid sponge habitat extent in the North Pacific, as well as climate-induced changes in oceanic dissolved oxygen levels and how this will impact sponges. Results from a MaxEnt model based on sponge presence data from the eastern Pacific, in conjunction with bathymetric terrain derivatives, closely mapped existing sponge habitats, and suggested a depth threshold around 3000 meters below which sponges are not found. Early results suggest that oxygen is another important predictor of sponge habitat, including this and a variety of other environmental predictors (e.g. based on ocean chemistry, physics and biology) and different model scales would improve model accuracy. The long-term goal of this research is to apply climate prediction data to the predictive modeling in order to assess the sensitivity of deep-sea sponge habitat to global climate changes.

Sign in / Sign up

Export Citation Format

Share Document