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Poster presentation

Multi-element analysis of discrete samples via X-Ray fluorescence or inductively coupled plasma spectrometry is commonly used to characterise the composition of solid geo-materials for environmental geochemical characterisation. Conventional geochemical analysis of individual samples is time consuming and costly, which often results in low-resolution sampling with the danger of missing crucial information. X-ray fluorescence Core Scanning (XCS) provides an alternative method to obtain elemental information, which can be potentially used quantitatively when combined with the Multivariate Log-ratio Calibration (MLC) approach. The suitability of the XCS-MLC method was tested for environmental geochemical characterisation on four terrestrial Holocene-Pleistocene sediment cores that have a variable lithology (clay, sand, peat, with variable calcareous content), were stored at ambient room conditions and scanned post sampling. Element contents based on XCS-MLC and conventional geochemical analysis proved to be comparable (R2 > 0.5) for Al, Ca, Cr, Fe, K, Sr, Mn, Ni, Pb, Rb, S, Si, Ti, Zn, Zr, and also for Br as proxy for organicmatter. For As, Cu and Ba the correlations were less satisfactory (R 2 < 0.4) partially due to the low concentration ranges present in these sediments. For some samples aberrant high values for Ca, Fe, S, and Zn were introduced by application of the MLC method due to extrapolation outside the MLC-calibrated range. Similar to the conventional element analyses the XCS-MLC approach has the ability to retrieve quantitative element contents and enable the calculation of mineral phases such as calcium carbonate and reactive iron species. Both conventional and XCS-MLC methods reproduced the average contents per sediment layer and mostly their ranges. Nevertheless, local features such as diagenetic enrichments were not always evident from the discrete samples. Thus, a better understanding of the spatial heterogeneity in geochemical and mineralogical contents within the sediment layers was obtained with the additional XCS-MLC data. Our study shows that also cores stored under unfavourable conditions can be reliably re-analysed with XCS to generate high resolution records.

Predicting which marine systems are close to abrupt transitions into oxygen-deficient conditions (“anoxia”) is notoriously hard but important—as rising temperatures and coastal eutrophication drive many marine systems toward such tipping points. Rapid oxic-to-anoxic transitions occurred regularly within the eastern Mediterranean Sea on (multi)centennial time scales, and hence, its sedimentary archive allows exploring statistical methods that can indicate approaching tipping points. The here presented high-resolution reconstructions of past oxygen dynamics in the Mediterranean Sea reveal that early-warning signals in these deoxygenation time series occurred long before fast transitions to anoxia. These statistical indicators (i.e., rise in autocorrelation and variance) are hallmarks of so-called critical slowing down, signaling a steady loss of resilience of the oxygenated state as the system approaches a tipping point. Hence, even without precise knowledge of the mechanisms involved, early-warning signals for widespread anoxia in marine systems are recognizable using an appropriate statistical approach.

X‐ray fluorescence (XRF) scanning of marine sediment has the potential to yield near‐continuous and high‐resolution records of elemental abundances, which are often interpreted as proxies for paleoceanographic processes over different time scales. However, many other variables also affect scanning XRF measurements and convolute the quantitative calibrations of element abundances and comparisons of data from different labs. Extensive interlab comparisons of XRF scanning results and calibrations are essential to resolve ambiguities and to understand the best way to interpret the data produced. For this study, we sent a set of seven marine sediment sections (1.5 m each) to be scanned by seven XRF facilities around the world to compare the outcomes amidst a myriad of factors influencing the results. Results of raw element counts per second (cps) were different between labs, but element ratios were more comparable. Four of the labs also scanned a set of homogenized sediment pellets with compositions determined by inductively coupled plasma‐optical emission spectrometry (ICP‐OES) and ICP‐mass spectrometry (MS) to convert the raw XRF element cps to concentrations in two ways: a linear calibration and a log‐ratio calibration. Although both calibration curves are well fit, the results show that the log‐ratio calibrated data are significantly more comparable between labs than the linearly calibrated data. Smaller‐scale (higher‐resolution) features are often not reproducible between the different scans and should be interpreted with caution. Along with guidance on practical calibrations, our study recommends best practices to increase the quality of information that can be derived from scanning XRF to benefit the field of paleoceanography., Geochemistry, Geophysics, Geosystems, 21 (9), ISSN:1525-2027

Utrecht University, The Netherlands (1); Université du Québec à Montréal, Canada (2); University of Geneva, Switzerland (3); Royal Netherlands Institute for Sea Research, and Utrecht University, The Netherlands (4; Geological Survey of Western Australia (5) Large-scale banded iron formations (BIFs) were deposited during Neoarchean to early Paleoproterozoic and have been mainly linked to hydrothermal plume activity and the rise of oxygen in the ocean and atmosphere. However, the potential influence of astronomical “Milankovitch” forcing on their deposition has been largely neglected so far and was hindered by large uncertainties in BIF depositional rates. In a recent study, we showed that outcrops of the ~2.5 Ga Kuruman Banded Iron Formation (BIF) in South Africa reveal a characteristic pattern in weathering profile that can be laterally traced over ~250 km (Lantink et al., 2019)*. Cyclostratigraphic analysis combined with high-precision U-Pb dating indicated that this pattern reflects a hierarchy of two superimposed cycles linked to the 405 kyr and a 1.2 - 1.6 Myr eccentricity cycle. This pattern is mirrored in magnetic susceptibility and high-resolution XRF core scan data of a new drill-core through the Kuruman BIF, providing insight into the chemical variations and may be used for cyclostratigraphic correlations to the broadly time-equivalent6 Australian Dales Gorge Member BIF. Furthermore, the dominance of eccentricity implies that precession-related cycles should be visible as well within the BIF stratigraphy and may be used to reconstruct the Precambrian evolution of the Earth-Moon distance. *https://doi.org/10.1038/s41561-019-0332-8 Lantink-Astronomical climate forcing of ~25 Ga banded iron formations_Info.pdf

Reconstructing millennial- to centennial-scale climate variability for the Eemian—an interval with estimated sea surface temperatures ~0.5 °C warmer than “preindustrial”—requires records with high temporal resolution. Sapropel S5 sediments, deposited under anoxic conditions in the Eastern Mediterranean Sea, offer the rare opportunity to assess multicentennial climate variability during this time. Here we present high-resolution S5 piston core data from the Nile delta region. Specifically, we focus on Ba/Ti, Br/Ti, and Mo/Ti, as they are proxies for paleo-productivity, marine organic carbon, and sediment anoxia, respectively. A high correlation between our Ba/Ti values in core 64PE-406-E1 and well-dated Ba records of nearby cores (LC21 and ODP967) was found. We, therefore, tuned our data to these cores obtaining an initial age model. A time-frequency analyses indicated significant frequency content in the multicentennial band, although the frequency components drifted over time. Assuming spectral simplicity, we corrected for sedimentation rate changes on a multicentennial time scale. This novel approach grants a higher-resolution age model. The resulting variability in sedimentation rate is similar to records of monsoon variability, indicating a possible link between sedimentation at the core location and low-latitude monsoon variability, linked via the River Nile. Moreover, the periodicities found in the sapropel time series are similar to the frequency content of total solar irradiance and sunspot records known for the Holocene, at least at high frequencies (~50–150 years). Hence, our data suggest cyclic intrasapropel variability, at least during the deposition of sapropel S5, may be linked to solar cycles.

Sapropels are organic-rich sediment layers deposited in the eastern Mediterranean Sea during precession minima, resulting from an increase in export productivity and/or preservation. Increased freshwater delivery from the African continent resulted in stratification, causing deepwater anoxia, while nutrient input stimulated productivity, presumably at the deep chlorophyll maximum. Previous studies have suggested that during sapropel deposition, nitrogen fixation was widespread in the highly stratified surface waters, and that cyanobacteria symbiotic with diatoms (diatom-diazotroph associations, DDAs) were responsible. Here we analyzed sapropel S5 sediments for heterocyst glycolipids (HGs) from three locations in the eastern Mediterranean. HG biomarkers can differentiate between those heterocystous cyanobacteria that are free living (found predominately in freshwater or brackish environments) and those that are from DDAs (found in marine settings). In our primary core, from a location which would have been influenced by the Nile River outflow, we detected a HG with a pentose (C5) head group specific for DDAs. However, HGs with a hexose (C6) head group, specific to free-living cyanobacteria, were present in substantially (up to 60×) higher concentration. These data suggest that at our study location, free-living cyanobacteria were the dominant diazotrophs, rather than DDAs. The C6 HGs increased substantially at the onset of sapropel S5 deposition, suggesting that substantial seasonal cyanobacterial blooms were associated with a brackish surface layer flowing from the Nile into the eastern Mediterranean. Two additional S5 sapropels were analyzed, one also from the Nile delta region and one from the region between Libya and southwestern Crete. Overall, comparison of the HG distribution in the three S5 sapropels provides evidence that all three locations were initially influenced by surface salinities that were sufficiently low to support free-living heterocystous cyanobacteria. While free-living heterocystous cyanobacteria continued to outnumber DDAs during sapropel deposition at the two Nile-influenced sites, DDAs, indicators of persistent marine salinities, were the dominant diazotrophs in the upper part of the sapropel at the more westerly site. These results indicate that N2 fixation by free-living cyanobacteria offers an important additional mechanism to stimulate productivity in regions with strong river discharge during sapropel deposition.

Eastern Mediterranean sapropels S1 (∼10.5–6.1 kyr BP) and S3 (∼85.8–80.8 kyr BP) formed respectively under full interglacial and glacial inception conditions. Consequently, the environmental factors preconditioning and leading to sapropel formation (e.g., global sea level and monsoonal forcing) were different. These factors must have differently influenced processes such as marine productivity, water column processes, and related seafloor anoxia. Here we investigate these differences through an interdisciplinary approach using dinoflagellate cyst and pollen/spore assemblages and sedimentary (redox-sensitive) trace-metal concentrations from a core in the central Nile delta area. Comparing S1 to S3, we demonstrate that (1) Nile discharge appears to be stronger during S3 than S1, as shown by δ18Oresiduals, higher ratio of pollen and spores, and the higher abundance of coastal dinocysts and freshwater palynomorphs, (2) Ba/Al, Corg, and dinocyst accumulation rates indicate that marine productivity was similar at least during the first phase of their deposition and started prior to the onset of both sapropels, (3) bottom water conditions were more reducing during S3, resulting in higher Mo/Al, S, and Corg/Ptot values, but preservation was high and similar for both sapropels, and (4) Sedimentary Mo- U covariation indicates that the depth of water-column ventilation during deposition of S3 was shallower than during S1 (∼1000 m versus ∼1800m, respectively). We attribute the observed differences to slightly enhanced precessional-forced monsoon intensity and potentially lower global sea level, resulting not only in increased North-African run-off, but also in reduced ventilation during S3 compared to S1.

The only low latitude pathway of heat and salt from the Pacific Ocean to the Indian Ocean, known as Indonesian Throughflow (ITF), has been suggested to modulate Global Mean Surface Temperature (GMST) warming through redistribution of surface Pacific Ocean heat. ITF observations are only available since ~1990s, and thus, its multidecadal variability on longer time scales has remained elusive. Here we present a 200 year bimonthly record of geochemical parameters (δ18O-Sr/Ca) measured on Cocos (Keeling) corals tracking sea surface temperature (SST; Sr/Ca) and sea surface salinity (SSS; seawater-δ18O═δ18Osw) in the southeastern tropical Indian Ocean (SETIO). Our results show that SETIO SSS and δ18Osw were impacted by ITF transport over the past 60 years, and therefore, reconstructions of Cocos δ18Osw hold information on past ITF variability on longer time spans. Over the past 200 years ITF leakage into SETIO is dominated by the interannual climate modes of the Pacific Ocean (El Niño—Southern Oscillation) and Indian Ocean (Indian Ocean Dipole). Pacific decadal climate variability (represented by the Pacific Decadal Oscillation) significantly impacted ITF strength over the past 200 years determining the spatiotemporal SST and SSS advection into the Indian Ocean on multidecadal time scales. A comparison of our SETIO δ18Osw record to GMST shows that ITF transport varied in synchrony with global warming rate, being predominantly high/low during GMST warming slowdown/acceleration, respectively. This hints toward an important role for the ITF in global warming rate modulation.