Using an independent geochronology based on palaeomagnetic secular variation (PSV) and atmospheric Pb deposition to date Baltic Sea sediments and infer 14C reservoir age

Dating of sediment cores from the Baltic Sea has proven to be difficult due to uncertainties surrounding the C-14 reservoir age and a scarcity of macrofossils suitable for dating. Here we present the results of multiple dating methods carried out on cores in the Gotland Deep area of the Baltic Sea. Particular emphasis is placed on the Littorina stage (8 ka ago to the present) of the Baltic Sea and possible changes in the C-14 reservoir age of our dated samples. Three geochronological methods are used. Firstly, palaeomagnetic secular variations (PSV) are reconstructed, whereby ages are transferred to PSV features through comparison with varved lake sediment based PSV records. Secondly, lead (Pb) content and stable isotope analysis are used to identify past peaks in anthropogenic atmospheric Pb pollution. Lastly, C-14 determinations were carried out on benthic foraminifera (Elphidium spec.) samples from the brackish Littorina stage of the Baltic Sea. Determinations carried out on smaller samples (as low as 4 mu g C) employed an experimental, state-of-the-art method involving the direct measurement of CO2 from samples by a gas ion source without the need for a graphitisation step - the first time this method has been performed on foraminifera in an applied study. The PSV chronology, based on the uppermost Littorina stage sediments, produced ten age constraints between 6.29 and 1.29 cal ka BP, and the Pb depositional analysis produced two age constraints associated with the Medieval pollution peak. Analysis of PSV data shows that adequate directional data can be derived from both the present Littorina saline phase muds and Baltic Ice Lake stage varved glacial sediments. Ferrimagnetic iron sulphides, most likely authigenic greigite (Fe3S4), present in the intermediate Ancylus Lake freshwater stage sediments acquire a gyroremanent magnetisation during static alternating field (AF) demagnetisation, preventing the identification of a primary natural remanent magnetisation for these sediments. An inferred marine reservoir age offset (Delta R) is calculated by comparing the foraminifera C-14 determinations to a PSV & Pb age model. This Delta R is found to trend towards younger values upwards in the core, possibly due to a gradual change in hydrographic conditions brought about by a reduction in marine water exchange from the open sea due to continued isostatic rebound. (C) 2012 Elsevier Ltd. All rights reserved

Recovery from multi‐millennial natural coastal hypoxia in the Stockholm Archipelago, Baltic Sea, terminated by modern human activity

Enhanced nutrient input and warming have led to the development of low oxygen (hypoxia) in coastal waters globally. For many coastal areas, insight into redox conditions prior to human impact is lacking. Here, we reconstructed bottom water redox conditions and sea surface temperatures (SSTs) for the coastal Stockholm Archipelago over the past 3000 yr. Elevated sedimentary concentrations of molybdenum indicate (seasonal) hypoxia between 1000b.c.e.and 1500c.e. Biomarker-based (TEX86) SST reconstructions indicate that the recovery from hypoxia after 1500c.e.coincided with a period of significant cooling (similar to 2 degrees C), while human activity in the study area, deduced from trends in sedimentary lead and existing paleobotanical and archeological records, had significantly increased. A strong increase in sedimentary lead and zinc, related to more intense human activity in the 18(th)and 19(th)century, and the onset of modern warming precede the return of hypoxia in the Stockholm Archipelago. We conclude that climatic cooling played an important role in the recovery from natural hypoxia after 1500c.e., but that eutrophication and warming, related to modern human activity, led to the return of hypoxia in the 20(th)century. Our findings imply that ongoing global warming may exacerbate hypoxia in the coastal zone of the Baltic Sea

Response of the Great Barrier Reef to sea level and environmental changes over the past 30,000 years

Previous drilling through submerged fossil coral reefs has greatly improved our understanding of the general pattern of sea-level change since the Last Glacial Maximum, however, how reefs responded to these changes remains uncertain. Here we document the evolution of the Great Barrier Reef (GBR), the world\u27s largest reef system, to major, abrupt environmental changes over the past 30 thousand years based on comprehensive sedimentological, biological and geochronological records from fossil reef cores. We show that reefs migrated seaward as sea level fell to its lowest level during the most recent glaciation (~20.5-20.7 thousand years ago (ka)), then landward as the shelf flooded and ocean temperatures increased during the subsequent deglacial period (~20-10 ka). Growth was interrupted by five reef-death events caused by subaerial exposure or sea-level rise outpacing reef growth. Around 10 ka, the reef drowned as the sea level continued to rise, flooding more of the shelf and causing a higher sediment flux. The GBR\u27s capacity for rapid lateral migration at rates of 0.2-1.5 m yr−1 (and the ability to recruit locally) suggest that, as an ecosystem, the GBR has been more resilient to past sea-level and temperature fluctuations than previously thought, but it has been highly sensitive to increased sediment input over centennial-millennial timescales

Orbital, the Box : an Interactive Educational Tool for In-depth Understanding of Astronomical Climate Forcing

“Orbital, the Box” provides an interactive tool with graphical user interface (GUI) for stimulating active, visual learning for understanding of astronomical climate forcing. This cross-platform tool can be run locally on a personal computer using a standard web browser environment with no need for plugins, thus maximising accessibility for students and teachers alike. The tool facilitates in the development of a holistic and quantitative understanding of astronomical climate forcing by allowing students to independently vary orbital parameters, after which they can instantaneously see the resulting effect upon the seasonal and latitudinal distribution of solar irradiance arriving at the top of the Earth’s atmosphere. Such an approach follows a classic controlled experimental design whereby one parameter can be changed while all others are kept constant. This experimental tool can be deployed as a virtual laboratory, including within a flipped classroom setting, to promote active learning of traditionally challenging concepts such as the roles of eccentricity and precession in astronomical climate forcing, and in particular their interaction with Kepler’s second law and the subsequent consequences for season length

USING SEDIMENTOLOGICAL PRIORS TO IMPROVE C-14 CALIBRATION OF BIOTURBATED SEDIMENT ARCHIVES

Radiocarbon (C-14) 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 existing C-14 calibration techniques were originally developed for age homogeneous material, such as archaeological artifacts or individual tree rings. 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 artifacts during periods of the Earth's history coinciding with highly dynamic atmospheric Delta C-14. Here, a new 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 C-14 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-optimized computer script (biocal) for the new calibration protocol is also presented, thus allowing for rapid and automated application of the new calibration protocol. This new calibration protocol could be applied within existing age-depth modeling software packages to produce more accurate geochronologies for bioturbated sediment archives

SEAMUS (v1.20) : a Δ14C-enabled, single-specimen sediment accumulation simulator

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