Ludvig Löwemark

Arctic Ocean manganese contents and sediment colour cycles

Cyclical variations in colour and manganese content in sediments from the central Arctic Ocean have been interpreted to represent climatically controlled changes in the input of Mn from the Siberian hinterland, and/or variations in the intermediate and deep water ventilation of the Arctic basins, although a diagenetic origin has not been excluded. A reinvestigation of core 96/12-1pc using an Itrax X-ray fluoresence (XRF) core scanner confirms that these colour cycles are indeed controlled by variations in Mn content, although changes in the source region of the sediment may override the Mn colour signal in certain intervals. The prominent Mn cycles show no correspondence to any of the other measured elements. This decoupling of Mn and the bulk chemistry of the sediment is taken to indicate that the cycles observed are caused by variations in water column ventilation and riverine input, rather than variations in sediment source or diagenesis. We therefore conclude that the Mn maxima do represent warm phases with increased ventilation and/or riverine input, and that they therefore could be used for chronostratigraphic correlation between cores from the central Arctic Ocean, where traditional isotope stratigraphy is difficult or impossible to establish because of the lack of calcareous microfossils.

Trace fossils as a paleoceanographic tool: evidence from Late Quaternary sediments of the southwestern Iberian margin

Biogenic structures in Late Quaternary sediments from the southwestern Iberian continental slope were studied by using X-ray images from two cores from 580 and 1750 m water depth. Eight different ichnocoenoses were observed: indistinct bioturbation, Planolites-dominated, Thalassinoides-dominated, Chondrites-dominated, Planolites and Thalassinoides-dominated, pyritized microburrows such as Trichichnus and ‘Mycellia’-dominated, Chondrites, Trichichnus, and ‘Mycellia’-dominated and Zoophycos. Variations of the ichnocoenoses within the cores show a striking correlation with climatically induced changes in the hydrographical regime, i.e. current strength and bathymetric position of the Mediterranean Outflow Water (MOW). Comparison of the response of the ichnocoenoses to changes in bottom-water conditions and substrate between the two cores studied indicate that bottom-water oxygenation and enrichment of particulate organic matter at the base of the MOW layer are the primary factors controlling the ichnocoenoses. The traces even recorded short-term climatic changes such as the Younger Dryas cold event. The spatial and temporal distribution patterns are in good agreement with earlier models of the MOW history, which gives reason to see a refinement of trace fossils as a complementary tool for paleoceanographical studies.

Enhanced Mediterranean-Atlantic exchange during Atlantic freshening phases

The Atlantic-Mediterranean exchange of water at Gibraltar represents a significant heat and freshwater sink for the North Atlantic and is a major control on the heat, salt and freshwater budgets of the Mediterranean Sea. Consequently, an understanding of the response of the exchange system to external changes is vital to a full comprehension of the hydrographic responses in both ocean basins. Here, we use a synthesis of empirical (oxygen isotope, planktonic foraminiferal assemblage) and modeling (analytical and general circulation) approaches to investigate the response of the Gibraltar Exchange system to Atlantic freshening during Heinrich Stadials (HSs). HSs display relatively flat W–E surface hydrographic gradients more comparable to the Late Holocene than the Last Glacial Maximum. This is significant, as it implies a similar state of surface circulation during these periods and a different state during the Last Glacial Maximum. During HS1, the gradient may have collapsed altogether, implying very strong water column stratification and a single thermal and δ18Owater condition in surface water extending from southern Portugal to the eastern Alboran Sea. Together, these observations imply that inflow of Atlantic water into the Mediterranean was significantly increased during HS periods compared to background glacial conditions. Modeling efforts confirm that this is a predictable consequence of freshening North Atlantic surface water with iceberg meltwater and indicate that the enhanced exchange condition would last until the cessation of anomalous freshwater supply into to the northern North Atlantic. The close coupling of dynamics at Gibraltar Exchange with the Atlantic freshwater system provides an explanation for observations of increased Mediterranean Outflow activity during HS periods and also during the last deglaciation. This coupling is also significant to global ocean dynamics, as it causes density enhancement of the Atlantic water column via the Gibraltar Exchange to be inversely related to North Atlantic surface salinity. Consequently, Mediterranean enhancement of the Atlantic Meridional Overturning Circulation will be greatest when the overturning itself is at its weakest, a potentially critical negative feedback to Atlantic buoyancy change during times of ice sheet collapse.

Recommendations for using XRF core scanning as a tool in tephrochronology

X-ray fluorescence (XRF) core scanning is a relatively new arrangement of a classic analytical technique which allows for non-destructive, in situ XRF analysis of sediment cores from submillimetre resolution upwards. In this contribution we explore the use of XRF core scanning for tephrochronology based on the analysis of three gyttja-rich sediment cores from the Faroe Islands. Using a combination of optical and radiographic images, analytical parameters and elemental profiles (Si, K, Ca, Ti, Mn, Fe, Sr and Zr), higher concentration basaltic tephra layers (>1000 shards/cm3) were positively identified. The XRF core scanning did not capture the lower concentration (<850 shards/cm3) rhyolitic layers found in the core. The elemental data generated for the detected tephra layers using XRF core scanning was not comparable to individual shard analysis by electron microprobe. We recommend using XRF core scanning for tephra screening in order to localize depths for high-resolution subsampling and to avoid depths where sediment mixing has caused tailing/mixing of the tephra signal. At the studied site the basaltic Saksunarvatn ash as well as a tephra belonging to the Askja-S/10 ka eruption were identified.

Reconstructing the southern South China Sea upper water column structure since the Last Glacial Maximum: Implications for the East Asian winter monsoon development

difference in the stable oxygen isotopes (Dd 18 O) and Mg/Ca‐based temperatures (DT) of surface‐dwelling (G. ruber) and thermocline‐dwelling (P. obliquiloculata) planktonic foraminifera and the temperature difference between alkenone‐ and P. obliquiloculata Mg/Ca‐based temperatures to estimate the upper ocean thermal gradient at International Marine Past Global Change Study (IMAGES) core MD01‐2390. Estimates of the upper ocean thermal gradient were used to reconstruct mixed layer dynamics. We find that our Dd 18 O estimates are biased by changes in salinity and, thus, do not display a true upper ocean thermal gradient. The D To fG. ruber and P. obliquiloculata as well as the alkenone and P. obliquiloculata suggest increased surface water mixing during the late glacial, likely due to enhanced EAWM winds. Surface water mixing was weaker during the late Holocene, indicating a weaker influence of winter monsoon winds. The weakest winter monsoon activity occurred between 6.5 ka and 2.5 ka. Inferred EAWM changes since the Last Glacial Maximum coincide with EAWM changes as recorded in Chinese loess sediments. We find that the intensity of the EAWM and the East Asian summer monsoon show an inverse behavior during the last glacial and deglaciation but covaried during the middle to late Holocene.

Temporal variations of the trace fossil Zoophycos in a 425 ka long sediment record from the South China Sea: implications for the ethology of the Zoophycos producer

A 425 ka long record from piston core GIK17925-3 taken in the northeastern South China Sea was used to study the environmental conditions controlling the distribution of the Zoophycos trace fossil. The 12 m long core offers a unique opportunity to study the response of the Zoophycos -producing animal to environmental variations over four glacial–interglacial cycles. The trace fossils show a strong glacial to interglacial variation in their abundance with a special preference for intervals with low sedimentation rates. Additional X-ray radiograph studies of piston cores from the Southeast Asian Marginal Seas show that the trace fossil Zoophycos is widespread in slope and deep-marine sediments, with the highest abundances encountered in low sedimentation rate settings. The preference of the Zoophycos producer for low sedimentation rates in a setting with strong seasonal fluctuations in food supply due to the shift between winter and summer monsoons, is interpreted to be the result of a cache-model behaviour, where food is collected during rich times and squirrelled away for poor times. Core GIK17925-3 also offers an opportunity to assess the impact of Zoophycos bioturbation on various palaeoenvironmental proxies. In this core, more than 30 % of the measured data points were more or less strongly affected by Zoophycos bioturbation. Together with the widespread occurrence this percentage indicates that Zoophycos may pose a serious threat to palaeoclimatic reconstructions in cores from low to moderate sedimentation rate sites.

Automatic image analysis of X-ray radiographs: a new method for ichnofabric evaluation

Abstract Automated ichnofabric analysis was performed on X-ray radiographs from five gravity cores retrieved from Late Quaternary sediments from the SW Portuguese continental slope. The image analysis program DIAna was used to detect and quantify the part of the ichnofabric consisting of pyritized microburrows, presumably Trichichnus and “ Mycellia ”. Abundance and orientation of the burrows were determined at 5-cm intervals. The results agree with manual counts of burrows in the coarse fraction of the sediment, and they indicate that the abundance of the pyritized burrows depends on enhanced organic carbon content, fine grain size, and low bottom water oxygenation. In contrast, no clear correlation between burrow orientation and environmental conditions was observed.

Biogenic and detrital-rich intervals in central Arctic Ocean cores identified using x-ray fluorescence scanning

X-ray fluorescence (XRF) scanning of sediment cores from the Lomonosov Ridge and the Morris Jesup Rise reveals a distinct pattern of Ca intensity peaks through Marine Isotope Stages (MIS) 1 to 7. Downcore of MIS 7, the Ca signal is more irregular and near the detection limit. Virtually all major peaks in Ca coincide with a high abundance of calcareous microfossils; this is particularly conspicuous in the cores from the central Arctic Ocean. However, the recorded Ca signal is generally caused by a combination of biogenic and detrital carbonate, and in areas influenced by input from the Canadian Arctic, detrital carbonates may effectively mask the foraminiferal carbonates. Despite this, there is a strong correlation between XRF-detected Ca content and foraminiferal abundance. We propose that in the Arctic Ocean north of Greenland a common palaeoceanographic mechanism is controlling Ca-rich ice-rafted debris (IRD) and foraminiferal abundance. Previous studies have shown that glacial periods are characterized by foraminfer-barren sediments. This implies that the Ca-rich IRD intervals with abundant foraminifera were most likely deposited during interglacial periods when glaciers left in the Canadian Arctic Archipelago were still active and delivered a large amount of icebergs. At the same time, conditions were favourable for planktic foraminifera, resulting in a strong covariance between these proxies. Therefore, we suggest that the XRF scanners capability to efficiently map Ca concentrations in sediment cores makes it possible to systematically examine large numbers of cores from different regions to investigate the palaeoceanographic reasons for the calcareous microfossils’ spatial and temporal variability.

Schaubcylindrichnus formosus isp. nov. in Miocene Sandstones from Northeastern Taiwan

Sheaves of smooth, thickly lined, vertically curved burrows occur abundantly in certain Tertiary strata in northern Taiwan. The burrows belong to the ichnogenus Schaubcylindrichnus, and they show several characteristics distinguishing them from previously established ichnospecies of this genus. The typical form in Taiwan is a sheaf converging into a crossing point in the lower part. This form is clearly different from the congruent sheaves and the loose bundles described for S. coronus and S. freyi, respectively. The observed Schaubcylindrichnus are significantly larger than other ichnospecies. The wide regional and stratigraphic spread of the ichnogenus Schaubcylindrichnus, together with the facies distribution ranging from shallow water to deep water settings and the distinct variation in morphology and size of the trace fossils motivate the proposition of a new species name, Schaubcylindrichnus formosus. The deposit feeder behavior proposed for other forms of Schaubcylindrichnus seems unlikely because of the successive, spreite-like overlap of burrows in the lower part of the system that would mean a repetitive exploitation of the same substrate. Schaubcylindrichnus formosus is interpreted as the work of a filter feeding worm living in offshore bar and shoreface environments.

Distribution and quantity of microplastic on sandy beaches along the northern coast of Taiwan

Plastic pollution is now ubiquitous in the worlds oceans, and studies have shown macroplastic and microplastic pollution of beaches in several East Asian countries. However, to our knowledge, no study of microplastic pollution has been conducted in Taiwan yet. Therefore, we collected sand samples from four beaches along the northern coast of Taiwan in 2015 and extracted microplastic particles using a saturated NaCl solution. Microplastic particles were identified using synchrotron-based FTIR spectroscopy. We recovered 4 to 532 particles from eight 0.0125m(3) samples, with a total of 1097 particles weighing 0.771g. A negative trend between the size of the particles and their numbers was documented. We thus established that microplastic pollution was ubiquitous along Taiwans northern coast. Future research should more comprehensively sample beaches around the entirety of Taiwans coast, and special emphasis should be placed on identifying different sources and movements of microplastic.