ALBERT

Description: The Labrador Sea is one of the key areas for deep water formation driving the Atlantic thermohaline circulation and thus plays an important role in Northern Hemisphere climatic fluctuations. In order to better constrain the overturning processes and the origins of the distinct water masses, combined dissolved Hf–Nd isotopic compositions and rare earth element (REE) distribution patterns were obtained from four water depth profiles along a section across the Labrador Sea. These were complemented by one surface sample off the southern tip of Greenland, three shallow water samples off the coast of Newfoundland, and two deep water samples off Nova Scotia. Although light REEs are markedly enriched in the surface waters off the coast of Newfoundland compared to north Atlantic waters, the REE concentration profiles are essentially invariant throughout the water column across the Labrador Sea. The hafnium concentrations of surface waters exhibit a narrow range between 0.6 and 1 pmol/kg but are not significantly higher than at depth. Neodymium isotope signatures (ɛNd) vary from unradiogenic values between −16.8 and −14.9 at the surface to more radiogenic values near −11.0 at the bottom of the Labrador Sea mainly reflecting the advection of the Denmark Strait Overflow Water and North East Atlantic Deep Water, the signatures of which are influenced by weathering contributions from Icelandic basalts. Unlike Nd, water column radiogenic Hf isotope signatures (ɛHf) are more variable representing diverse weathering inputs from the surrounding landmasses. The least radiogenic seawater ɛHf signatures (up to −11.7) are found in surface waters close to Greenland and near the Canadian margin. This reflects the influence of recirculating Irminger Current Waters, which are affected by highly unradiogenic inputs from Greenland. A three to four ɛHf unit difference is observed between Denmark Strait Overflow Water (ɛHf ∼ −4) and North East Atlantic Deep Water (ɛHf ∼ −0.1), although their source waters have essentially the same ɛNd signature. This most likely reflects different weathering signals of hafnium delivered to Denmark Strait Overflow Water and North East Atlantic Deep Water (incongruent weathering of old rocks from Greenland versus basaltic rocks from Iceland). In addition, the ɛHf data resolve two layers within the main body of Labrador Sea Water not visible in the ɛNd distribution, which are shallow Labrador Sea Water (ɛHf ∼ −2) and deep Labrador Sea Water (ɛHf ∼ −4.5). The latter layer was formed between the late 1980’s and mid 1990’s during the last cold state of the Labrador Sea and underwent substantial modification since its formation through the admixture of Irminger Water, Iceland Slope Water and North East Atlantic Deep Water, which is reflected in its less radiogenic ɛHf signature. The overall behavior of Hf in the water column suggests its higher sensitivity to local changes in weathering inputs on annual to decadal timescales. Although application of Hf isotopes as a tracer for global water mass mixing is complicated by their susceptibility to incongruent weathering inputs they are a promising tracer of local processes in restricted basins such as the Labrador Sea.

Description: Despite a clear correlation of alkenone unsaturation and sea surface temperatures (SST) throughout most parts of the ocean, scatter of the regression for various calibration equations has been shown to increase significantly at low SSTs. In this study, we combine previously published (n = 101) and new (n = 51) surface sediment data from the northern North Atlantic to constrain uncertainties of alkenone paleothermometry at low SSTs and to discuss possible sources of the increased scatter in the regression. The correlation between alkenone unsaturation and SSTs is strongest, in particular at the cold end (SSTs 〈 10°C), when the tetra-unsaturated alkenones (C37:4) are included in the unsaturation index (expressed as inline image) and regressed against spring-summer temperature. Surface ocean salinity and sea ice cover are not correlated with inline image per se. However, samples located in regions of permanent winter sea ice cover exhibit a significant warm bias. Deviation from the linear regression is posited to be related to a number of additional non-exclusive factors, such as advection of allochthonous material, local temperature stratification, and uncertainty in the absolute age of surface sediment samples assumed to be equivalent to modern conditions. We conclude that alkenone unsaturation allows accurate reconstruction of SST records from many regions of the North Atlantic if the factors confounding alkenone paleothermometry detailed here can be excluded.

Description: This Master thesis traces weathering input into the Kara Sea via the Yenisei and Ob Rivers, as the main suppliers for suspended and dissolved matter. With the use of Sr and Nd isotopes as tracers, potential sources of weathered material were complied for each river from the literature and based on previously published work in this area possible ranges in Sr and Nd isotopes for each potential source were derived, which allowed to reconstruct the radiogenic isotope evolution of dissolved and particulate inputs into the Kara Sea. In this study, the signatures of these two radiogenic isotopes systems were analyzed in surface samples and two cores: one core BP99-04/7 from the Yenisei estuary area and one core BP00-07/7 from southern part of the Kara Sea. All samples were prepared in the clean laboratory using ion chromatography and were then measured on a Multi Collector-Inductively Coupled Plasma Mass Spectrometer (MC-ICP-MS). The obtained values were compared to the signatures of possible sources to evaluate their origin. Fluctuations within both cores were analyzed and compared. Ages were calculated for each depth interval based on the chronologies of Polyakova and Stein (2002). Furthermore, to determine, if there is any correlation between the variability of the signatures and changes in climatic conditions, the data were compared to the GISP-2 ice core from Greenland, as well as changes in temperature and precipitation over the Taimyr Peninsula over the past 10,000 years, changes in salinity in the Kara Sea and changes in precipitation over the Lake Baikal. The main goal of this master thesis was to identify the possible sources of input of weathered material into the Kara Sea from land and the relationship between weathering inputs via river discharge and climate change through Holocene times based on the Sr and Nd radiogenic isotope composition in Fe-Mn coatings and totally dissolved detrital material.

Description: The Labrador Sea plays a crucial role in influencing the strength of the Atlantic Meridional Overturning Circulation (AMOC), as it is one of the main deep water formation sites. Numerous studies have been carried out in the Labrador Sea, covering different aspects and time scales. This thesis presents an investigation of the complex processes that prevail in the Labrador Sea covering surface, intermediate and deep waters using a combination of organic (alkenone paleothermometry) and inorganic (radiogenic isotopes) geochemical proxies. Over the past decades it has been debated, whether or not the alkenone unsaturation ratio can reliably be used as a paleo sea surface temperature proxy, especially in cold environments. Many studies have been published presenting contradicting results. In Chapter 4 an investigation of the proxy reliability was carried out based on available and newly obtained data from the western North Atlantic region. The results show that if certain conditions are met, such as appropriate age control of sediment samples, sufficient distance from the main ocean frontal systems characterized by high SST gradients, low risk of allochthonous input, alkenone-derived temperatures can be reliably used even in the cold regions of the North Atlantic. These results were applied to reconstruct sea surface temperatures in the Labrador Sea over the last 35 kyrs based on alkenone unsaturation ratio of marine sediment samples from four cores (Chapter 5). The motivation of this study was to reconstruct the variability of sea surface conditions in the area and correlate it to presence or absence of cold/warm surface water currents. The sediment cores used in this study were recovered from around the Labrador Sea following the flow path of the main alongshore currents. Prior to 10 ka the record was only partially preserved and indicated significantly cold temperatures and negative 𝑈37𝐾 values, suggesting that these data are unreliable. However, alkenone unsaturation ratios provided reliable sea surface temperature estimations over the Holocene. The study showed that the Holocene was a period of unstable climate in the Labrador Sea area, characterized by multiple temperature fluctuations that were caused by increased cold water input from the Arctic or fresh water inputs from land. The investigation of the intermediate and deep waters in the Labrador Sea in this thesis was based on the Hf-Nd-Pb radiogenic isotope signatures of various archives such as sediment samples, foraminifera, detrital material and seawater. The named above isotopes have been used as a proxies for water mass mixing and weathering inputs over the past two decades and were shown to be a reliable tool in paleoceanography. The Labrador Sea is a complex and dynamic region where the depth of the water formation varies from year to year. To be able to resolve these changes in the past more information is needed concerning mechanisms and processes crucial for the formation of the present day Hf-Nd isotope signatures of different water masses. In Chapter 3 new data were obtained from the Labrador Sea based on direct measurements of seawater. The results showed that all water masses present in the Labrador Sea have distinct hafnium and neodymium isotope signatures. However, some of the water masses such as the Irminger water and shallow and deep Labrador Sea Water are more distinctive in their hafnium isotope signatures compared to those of neodymium. This study suggests that ɛHf signatures of the main water masses in the Labrador Sea were most likely mainly formed due to weathering inputs from the surrounding terrains. Higher variability of ɛHf signatures suggests that its residence time is significantly shorter that previously assumed. The new data allow us to conclude that although tracing of large scale ocean mixing processes may not be possible based on Hf isotopes, there is clearly prospect for their application in other restricted basins with similar geological and hydrographic settings. Based on the above understanding in Chapter 6 combined Hf, Nd and Pb isotopes were applied for the reconstruction of the intermediate and deep water mass circulation in the Labrador Sea over the last 35 kyrs. The results show that all three isotope proxies could be reliably extracted from marine sediments. The new data suggest an early inception of the Denmark Strait Overflow Water around 12 ka, based on ɛNd and ɛHf signatures of the leachates and detrital fraction. The ɛHf and ɛNd signatures of the detrital fraction suggest the establishment of the alongshore Labrador current around 12 ka. The modern day circulation patterns were most likely absent prior 8 ka. Combined Hf-Nd-Pb isotope signatures suggest that convection during the late Holocene in the Labrador Sea may have been more intensive and formation of the Labrador Sea Water reached significantly deeper than today to up to 2600 m. The position of the new data on ɛHf-ɛNd plots forms its own new trend termed “the Labrador Sea” array, which could be representative of the weathering of particular rocks in the source areas of the waters around the Labrador Sea.

Description: Highlights • Sea-ice cover limits the accumulation of both coccoliths and alkenones in sediments. • Calcite dissolution in shelf sediments may explain the accumulation of alkenones in the presence of few or no coccoliths. • Non-calcifying haptophytes most likely produce alkenones in nearshore environments. Abstract We determined the abundances and concentrations of coccoliths and alkenones in 66 surface sediment samples from the northwest North Atlantic to evaluate the role that surface ocean temperature, salinity, sea-ice cover, and productivity have on the regional distribution of these two biological remains produced by haptophytes in the photic zone. In areas with sea-ice cover of more than 1 month per year, coccolith and alkenone concentrations in sediments are extremely low to nil. Elsewhere, the distribution of coccolith taxa generally displays strong relationships to water temperature, salinity, and productivity. Coccolithus pelagicus is associated with low summer sea-surface temperatures (〈8°C) and relatively high summer sea-surface salinities (〉33.5), whereas Helicosphaera carteri seems to follow the path of the North Atlantic Drift. The distribution of Emiliania huxleyi, the dominant alkenone producer, is not strongly correlated with that of alkenones. Calcite dissolution in shelf sediments could explain the occurrence of alkenones in the absence of coccoliths but alkenone production by non-calcifying haptophytes seems to also exert some control on alkenone concentrations in surface sediments, thus blurring alkenone abundance links to coccolithophorid production and their relative preservation.

Description: Limited constraints on the variability of the deep-water production in the Labrador Sea complicate reconstructions of the strength of the Atlantic Meridional Overturning Circulation (AMOC) during the Late Quaternary. Large volumes of detrital carbonates were repeatedly deposited in the Labrador Sea during the last 32 kyr, potentially affecting radiogenic Nd isotope signatures. To investigate this the Nd isotope compositions of deep and intermediate waters were extracted from the authigenic Fe-Mn oxyhydroxide fraction, foraminiferal coatings, the residual silicates and leachates of dolostone grains. We provide a first order estimation of Nd release via dissolution of detrital carbonates and its contribution to the authigenic ԑNd signatures in the Labrador Sea. During the Last Glacial Maximum the Nd isotope signatures in the Labrador Sea would allow active water mass mixing with more radiogenic ɛNd values (-12.6 and -14) prevailing in its eastern part whereas less radiogenic values (ɛNd ∼ -18.4) were found on the western Labrador slope. The deposition of detrital carbonates during Heinrich stadials (2,1) was accompanied by negative detrital and authigenic Nd isotope excursions (ɛNd ∼ -31) that were likely controlled by dissolution of dolostone or dolostone associated mineral inclusions. This highly unradiogenic signal dominated the authigenic phases and individual water masses in the Labrador Sea, serving as potential source of highly unradiogenic Nd to the North Atlantic region, while exported southward. The Holocene authigenic ɛNd signatures of the coatings and leachates significantly differed from those of the detrital silicates, approaching modern bottom water mass signatures during the Late Holocene. Key Points - Estimation of Nd release via dissolution of detrital carbonates and its contribution to the authigenic ԑNd signatures in the Labrador Sea - Dissolution of detrital dolostones in the water column during Heinrich stadials at least partially controlled ɛNd signatures - During the LGM generally more radiogenic signatures possibly indicate active water mass advection and mixing in the Labrador Sea

Description: Highlights • Porewater calcite dissolution may have occurred during the deglacial interval. • There is a significant decoupling of coccolith and alkenone concentrations in core 004. • Non-calcifying haptophytes most likely produced the alkenones in the glacial interval. Abstract The important changes that took place in the glacial cycle at the termination, from the Last Glacial Maximum to the present interglacial, deserve an examination of ocean sedimentary records that document past productivity, carbon fluxes, and carbonate preservation. In this study, we analyzed coccoliths, alkenones, and foraminifers in core HU2008–029-004 PC (61.46°N and 58.04°W, water depth = 2,674 m) from the northwestern Labrador Sea to document linkages between hydrographic conditions, biogenic carbonate fluxes to the seafloor, and their preservation/dissolution during the last 25,000 years. Large changes in coccolith and foraminifer concentrations are recorded, with sediments from the last glacial interval containing significantly less carbonate microfossils (9.5 ± 3.9 × 105 coccoliths g−1 and 2,860 ± 580 planktonic foraminifers g−1) than sediments from the deglacial and postglacial intervals (up to 3.1 × 108 coccoliths g−1 and 2.9 × 104 foraminifers g−1). Three foraminifer-based calcite dissolution indices were used to evaluate biogenic carbonate preservation: the planktonic foraminifer fragmentation index, the ratio of benthic-to-planktonic foraminifers (B/P), and the ratio of organic linings to benthic foraminifers (OL/B). Fragmentation remained low throughout the postglacial (mean of 4%) but reached up to 8% in the deglacial and peaked at 16% in samples from the Bølling-Allerød of the late glacial interval. Samples from the Bølling-Allerød and the deglacial interval also display a slightly elevated B/P index (〉0.15), which suggests that some dissolution may have occurred. In contrast, with the exception of the Bølling-Allerød and the deglacial interval, near zero OL/B values characterize most of the sequence, suggesting good biogenic carbonate preservation, which implies that the low biogenic carbonate and coccolith content in sediments of the glacial stage mirror low productivity of calcifying organisms. The elevated fragmentation of foraminifers during the Bølling-Allerød and the deglacial interval, a time of elevated productivity and low percentages of ice-rafted debris, may indicate the development of calcite undersaturated porewaters and consequent dissolution resulting from oxic remineralization of sedimentary organic matter. We also identify a significant decoupling of coccolith and alkenone concentrations throughout the core. Colder-than-expected UK37-SST estimates from the alkenones of the glacial interval rule out possible allochthonous inputs from lower-latitude locations. Instead, our records imply that at least during the glacial interval, alkenones were produced by non-calcifying haptophytes that may not follow the canonical UK37-based temperature calibrations.