Past changes in riverine input and ocean circulation in the Gulf of Guinea

Large river systems draining the West African Monsoon area deliver sediments and dissolved trace elements into the Gulf of Guinea (GoG) in the easternmost equatorial Atlantic. The different catchment areas of these river systems are characterized by different geological ages and rock types releasing distinct radiogenic neodymium isotope compositions during weathering which are supplied to the GoG. The main rivers discharging into the GoG are the Niger, the Sanaga, the Nyong and the Ntem with present day εNd signatures of -10.5 [1], -12.3, -12.5 and -28.1 [2], respectively. These riverine inputs mix with the tropical Atlantic surface waters. At intermediate water depths Antarctic Intermediate Water (AAIW) prevails whereas the deep basin at this location is mainly filled with NADW. We focus on a marine sediment core that was recovered off the Sanaga and Ntem Rivers and we reconstruct changes in riverine inputs and in mixing of surface and deep water masses over the past 140,000 years. Changes in riverine inputs most likely reflecting latitudinal shifts of the rainfall zones across the different catchment areas were obtained from the Nd isotope signatures of the residual detrital fraction of the sediment. Sediment leachates of several GoG core top samples reflect the riverine input from nearby rivers indicating transport of particles coated in the rivers. Both the sediment leachates and the residual detrital fraction show similar patterns, with shifts towards radiogenic values during the interglacials and least radiogenic values during glacial periods. This shift in εNd values may be attributed to the migration of the rainfall zones towards the north during interglacial times and thus implies the increased influence of the northern rivers, the Sanaga and Nyong. The oxidatively-reductively cleaned planktonic foraminiferal calcite of the core top samples in the GoG reflects surface seawater signatures. Non-reductively cleaned planktonic foraminiferal tests and cleaned shallow endo-benthic and epi-benthic foraminiferal tests were used to acquire information about past bottom waters. Difficulties in cleaning down core foraminiferal samples were experienced and these samples appear to be contaminated by secondary manganese and iron bearing phases, even after cleaning. Those phases may have overprinted the original surface water Nd isotope composition in the planktonic foraminiferal tests. As the planktonic and benthic foraminiferal values are overall similar to the sediment leachates, the foraminiferal isotope signatures are most likely overprinted by isotopic signals originating from the rivers due to remobilization processes in the sediments and formation of secondary phases such as Mn-carbonates, which are attached to the foraminiferal calcites. References: [1] Goldstein et al. (1984) Earth and Planetary Science Letter 70, 221-236. [2] Weldeab et al. (2011) Geophysical Research Letter 38, pp. 5

Trace Metals and Their Isotopes in the Tropical Atlantic Ocean - Cruise No. M81/1, February 04 – March 08, 2010, Las Palmas (Canary Islands, Spain) – Port of Spain (Trinidad & Tobago)

Summary Meteor Cruise M81/1 was dedicated to the investigation of the distribution of dissolved and particulate trace metals and their isotopic compositions (TEIs) in the full water column of the tropical Atlantic Ocean and their driving factors including main external inputs and internal cycling and ocean circulation. The research program is embedded in the international GEOTRACES program (e.g. Henderson et al., 2007), which this cruise was an official part of and thus corresponds to GEOTRACES cruise GA11. This cruise was completely dedicated to the trace metal clean and contamination-free sampling of waters and particulates for subsequent analyses of the TEIs in the home laboratories of the national and international participants. Besides a standard rosette for the less contaminant prone metals, trace metal clean sampling was realized by using a dedicated and coated trace metal clean rosette equipped with Teflon-coated GO-FLO bottles operated via a polyester coated cable from a mobile winch that was thankfully made available by the U.S. partners of the GEOTRACES program for this cruise. The particulate samples were also collected under trace metal clean conditions using established in-situ pump systems. The cruise track led the cruise southward from the Canary Islands to 11°S and then continued northwestward along the northern margin of South America until it reached Port of Spain, Trinidad & Tobago. The track crossed areas of major external inputs including exchange with the volcanic Canary Islands, the Saharan dust plume, as well as the plume of the Amazon outflow. In terms of internal cycling the equatorial high biological productivity band, as well as increased productivity associated with the Amazon Plume were covered. All major water masses contributing the Atlantic Meridional Overturning Circulation, as well as the distinct narrow equatorial surface and subsurface east-west current bands were sampled. A total of 17 deep stations were sampled for the different dissolved TEIs, which were in most cases accompanied by particulate sampling. In addition, surface waters were continuously sampled under trace metal clean conditions using a towed fish

The Late Quaternary evolution of subsurface water masses in the Gulf of Guinea: High-latitude versus monsoonal control

The area of this study has been under the influence of a monsoon-driven tropical climate and has experienced variable contributions from river systems in a near shelf area characterized by a complex hydrography. The goal of this study is to improve the understanding of the coupling mechanisms between high latitudes and the tropics via atmospheric and oceanic circulation. The study investigates subsurface tropical water masses and their role in hydrographic changes in response to bipolar climatic oscillations and in modulating the inter-hemispheric heat exchange, focussing on glacial-interglacial cycles and shorter term millennial scale variability, in particular during Marine Isotope Stage 3 (MIS 3). MIS 3 was characterized by millennial-scale temperature oscillations, which were essentially in opposite phase in the northern and southern hemisphere. Various geochemical proxies are investigated and tested for their reliability for distinct applications, such as reconstructing past riverine input, water mass mixing and boundary exchange. The radiogenic neodymium (Nd) isotope composition is a widely used proxy to investigate past changes in sources and mixing of water masses, in particular that extracted from foraminiferal shells. However, the extracted seawater Nd isotope ratios can be biased by con-taminant phases such as organic matter, detrital silicates, and early diagenetic ferromanganese coatings. We tested different cleaning methods (Flow Through and batch cleaning) of planktonic foraminifera in order to obtain seawater εNd signatures to distinguish surface water mass mixing from riverine inputs and deep water signatures. Both methods reveal indistinguishable levels of cleaning efficiency and identical Nd isotope compositions. Element/calcium ratios and rare earth element (REE) concentration patterns, as well as similar εNd ratios for cleaned and uncleaned foraminiferal samples, suggest that the planktonic foraminiferal Nd isotope signatures reflect bottom water or sedimentary pore water signa-tures. The results of planktonic foraminiferal analyses were complemented by analyses of Fe-Mn coatings of de-carbonated bulk sediment leachates and the residual detrital fractions of the same sediment samples. Close to the Niger River mouth, the sediment leachates differ significantly from the foraminiferal data, but agree well with the Niger River signature, sug-gesting contributions of pre-formed coatings originating from the river. The riverine input influences the geochemistry of seawater and deposited authigenic phases. The riverine inputs in the study area have been strongly influenced by the West African Monsoon (WAM) sys-tem, which itself has been closely linked to location and movement of the Intertropical Convergence Zone (ITCZ) and the related rain belt, resulting in changes of their discharge into the Gulf of Guinea. The evolution of the riverine inputs of two large river systems (Sanaga/Nyong and Ntem Rivers) during the past 135 kyr was reconstructed using different sedimentary phases (foraminiferal shells, bulk sediment leachates, residual detrital fraction) of core MD03-2707 which were analyzed for Nd isotope compositions, element/calcium ratios and REE concentrations. The isotopic signature of the residual detrital fraction directly reflects changes in local riverine input, whereas the sediment coatings and foraminiferal shells represent past bottom water signatures, as well as exchange processes between bottom water and the shelf and/or continental material. All phases show similar trends with markedly less radiogenic values during glacial phases as a consequence of the glacial southward migration of the ITCZ and the related rain belt resulting in enhanced runoff of more southerly located rivers draining Precambrian basement with characteristically unradiogenic Nd isotope signatures during sea level low stands. Combined measurements of trace element contents (Mg/Ca, Ba/Ca) and stable isotopes (δ18O, δ13C) of foraminiferal samples are applied to obtain a detailed record of the evolution of subsurface water masses (< 500 m) and their relationship to climate changes in high latitudes during the last glacial period (5 to 60 kyr). For this purpose, a core top-based multi-species Mg/Ca-temperature calibration of foraminiferal Mg/Ca and oxygen isotopes (δ18O) was established. Shells of two planktonic foraminiferal species N. dutertrei and G. crassaformis inhabiting different subsurface water masses the Subtropical Underwater (STUW) and South Atlantic Central Water (SACW), respectively, were investigated in detail. N. dutertrei represents the lower thermocline and the variability of its proxy data shows clear influence of northern hemisphere climate changes but was also affected by changes in the southern hemisphere due to enhanced admixture of water masses during the last glacial period. G. crassaformis inhabits a deeper water mass (SACW), which has mainly been affected by climate changes in the southern hemisphere. The similarity of the records of N. dutertrei and G. crassaformis, particularly between 55 and 32 kyr, suggests that the SACW strongly influ-enced the thermocline waters. Furthermore, vertical mixing most likely driven by stronger trade winds resulted in a decrease of thermocline depth during this period of time. Since 32 kyr the trends in the records of N. dutertrei and G. crassaformis have diverged, suggesting a decrease of the influence of southern water masses and an attenuation of the mixing of the thermocline and SACW water masses.Das Untersuchungsgebiet unterliegt einem monsungesteuerten tropischen Klima und ist durch hohe Einträge der Flusssysteme in einem schelfnahen Gebiet beeinflusst, was in einer komplexen Hydrographie resultiert. Ziel dieser Studie ist es, das Verständnis der Kopplungsmechanismen zwischen hohen Breiten und den Tropen über atmosphärische und ozeanische Zirkulation zu verbessern. Sie befasst sich mit den Wassermassen oberhalb von 500 m, den hydrographischen Veränderungen als Reaktion auf die bipolaren Klimaschwankungen, sowie deren regulierende Rolle für den inter-hemisphärischen Wärmeaustausch. Der Fokus liegt hierbei auf dem Marinen Isotopenstadium 3, das durch Klima-Zyklen auf Zeitskalen von Tausenden von Jahren geprägt war, die sich in der Nord- und Südhemisphäre gegenläufig verhielten. Verschiedene geochemische Proxy-Indikatoren wurden untersucht und auf ihre Verlässlichkeit für Anwendungen, wie der Rekonstruktion von Flusseinträgen, Wassermassenmischung und von Austauschprozessen mit den Schelfsedimenten getestet. Ein häufig verwendeter Proxy um Veränderungen des Ursprungs und in der Mischung von Wassermassen zu rekonstruieren sind die radiogenen Nd-Isotopen-Verhältnisse (εNd). Die aus den Foraminiferen extrahierten Meerwasser-εNd-Signaturen können durch organisches Material, detritische Silikate und frühdiagenetische Eisen-Mangan-Oxide überprägt worden sein. Zwei Reinigungsmethoden ("Flow Through" und "batch cleaning") wurden untersucht und verglichen, um die Effektivität der Methoden und die Verlässlichkeit der extrahierten εNd-Verhältnisse zu bewerten. Elementverhältnisse (Al/Ca, Mn/Ca) und identische εNd-Werte (der gleichen Probe) implizieren, dass die Effektivität beider Reinigungsmethoden gleichwertig ist. Die Elementverhältnisse und die Verteilungsmuster der Seltenen Erdelemente (REE), sowie identische Nd-Isotopensignaturen gereinigter und ungereinigter Foraminiferen, weisen darauf hin, dass die εNd-Signaturen der planktonischen Foraminiferen Boden- oder Porenwässer repräsentieren. Die Ergebnisse der Foraminiferen-Analysen wurden mit Messungen von Eisen-Mangan-Oxiden von entkarbonatisierten Sedimentproben und der detritischen Fraktion des selben Sediments ergänzt. Nahe der Nigermündung, weichen die Nd-Ergebnisse der Fe-Mn-Oxide signifikant von den Foraminiferen-Daten ab, sie stimmen jedoch mit den Niger-Signaturen überein, was auf einen signifikanten Beitrag von im Fluss vorgeformten Fe-Mn-Oxiden schließen lässt. Flusseinträge können die Geochemie des Meerwassers sowie der authigenen Ablagerungen beeinflussen. Der Flusseintrag im Untersuchungsgebiet wird stark vom Westafrikanischen Monsunsystem beeinflusst, das eng mit der Lokation und der Oszillation der Intertropischen-Konvergenz-Zone (ITCZ) über die verschiedenen Breitengrade und dem damit verbundenen Regenband gekoppelt ist. Diese Migration über die Einzugsgebiete der Hauptflüsse führt folglich zu Variationen im Einstrom in den Golf von Guinea. Der Flusseintrag der zwei großen Flusssysteme (Sanaga/Nyong und Ntem) während der letzten 135 ka wurde mit Hilfe verschiedener Sedimentphasen (Foramini-feren, Sediment-Coatings, detritische Sedimentfraktion) aus dem Kern MD03-2707 rekon-struiert. Es wurden dazu Nd-Isotope, Elementverhältnisse und REE-Konzentrationen untersucht. Die detritische Fraktion zeigt direkt Veränderungen im lokalen Flusseintrag an, wohin-gegen die Fe-Mn-Oxide des Sediments und die Foraminiferen Paläo-Bodenwasser-Signaturen und Austauschprozesse zwischen Bodenwasser und Schelf-Material und/oder kontinentalem Material repräsentieren. Alle analysierten Phasen zeigen ähnliche Trends mit markant unradiogenen Nd-Isotopenwerten während der glazialen Perioden als Konsequenz der südwärts Migration der ITCZ und dem Regenband, was in einem erhöhten Abfluss aus südlicheren Gebieten mit präkambrischem Untergrund (unradiogene εNd-Signaturen) resultierte. Mit Hilfe kombinierter Messungen von Spurenelementverhältnissen (Mg/Ca, Ba/Ca) und stabilen Isotopen (δ18O, δ13C) an Foraminiferen wurde die Entwicklung der Wassermassen oberhalb von 500 m Wassertiefe, ebenso wie deren Beziehung zu Klimaveränderungen in hohen Breiten mit Fokus auf dem letzten Glazial (5 bis 60 ka) untersucht. Hierfür wurde eine oberflächenbasierte Multispezies-Mg/Ca-Temperatur-Kalibration mit Hilfe von Mg/Ca-Verhältnissen und Sauerstoffisotopen (δ18O) erstellt. Zwei Spezies aus verschiedenen Habitat-Tiefen wurden für die Analysen ausgewählt. N. dutertrei kommt im Subtropischen Unterwasser (STUW) vor und repräsentiert die untere Thermokline, welche sowohl Einflüsse aus der Nordhemisphäre, als auch aus der Südhemisphäre während des letzten Glazials wiederspiegelt. G. crassaformis bewohnt dagegen tiefere Wasserschichten und repräsentiert hauptsächlich den Einfluss des Südatlantische Zwischenwassers (SACW), das durch Klima-schwankungen der Südhemisphäre geprägt ist. Die signifikante Korrelation der Ergebnisse beider Spezies, besonders zwischen 55 und 32 ka, legt nahe, dass das SACW die Thermokline wahrscheinlich durch Mischungsprozesse stark beeinflusst, die außerdem eine Verflachung der Thermokline verursachten und durch starke Passatwinde ausgelöst worden sein können. Seit 32 ka laufen die Kurven von N. dutertrei und G. crassaformis auseinander, was für eine Abschwächung des Einflusses der südlichen Wassermassen auf die Thermokline und eine Abschwächung der Mischungsprozesse der Wassermassen spricht

Past changes in riverine input in the Gulf of Guinea - linked to the migration of the ITCZ

Large river systems draining the West African Monsoon area deliver sediments and dissolved trace elements into the Gulf of Guinea (GoG) in the easternmost equatorial Atlantic. The different catchment areas of these river systems are characterized by different geological ages and rock types releasing distinct radiogenic neodymium isotope compositions during weathering which are supplied to the GoG. The main rivers discharging into the GoG are the Niger, the Sanaga, the Nyong and the Ntem with present day εNd signatures of -10.5 [1], -12.3, -12.5 and -28.1 [2], respectively. These riverine inputs mix with the tropical Atlantic surface waters. At intermediate water depths Antarctic Intermediate Water (AAIW) prevails whereas the deep basin at this location is mainly filled with NADW. We focus on a marine sediment core that was recovered off the Sanaga and Ntem Rivers and we reconstruct changes in riverine inputs and in mixing of surface and deep water masses over the past 140,000 years. Changes in riverine inputs most likely reflecting latitudinal shifts of the rainfall zones across the different catchment areas were obtained from the Nd isotope signatures of the residual detrital fraction of the sediment. Sediment leachates of several GoG core top samples reflect the riverine input from nearby rivers indicating transport of particles coated in the rivers. Both the sediment leachates and the residual detrital fraction show similar patterns, with shifts towards radiogenic values during the interglacials and least radiogenic values during glacial periods. This shift in εNd values may be attributed to the migration of the rainfall zones towards the north during interglacial times and thus implies the increased influence of the northern rivers, the Sanaga and Nyong. The oxidatively-reductively cleaned planktonic foraminiferal calcite of the core top samples in the GoG reflects surface seawater signatures. Non-reductively cleaned planktonic foraminiferal tests and cleaned shallow endo-benthic and epi-benthic foraminiferal tests were used to acquire information about past bottom waters. Difficulties in cleaning down core foraminiferal samples were experienced and these samples appear to be contaminated by secondary manganese and iron bearing phases, even after cleaning. Those phases may have overprinted the original surface water Nd isotope composition in the planktonic foraminiferal tests. As the planktonic and benthic foraminiferal values are overall similar to the sediment leachates, the foraminiferal isotope signatures are most likely overprinted by isotopic signals originating from the rivers due to remobilization processes in the sediments and formation of secondary phases such as Mn-carbonates, which are attached to the foraminiferal calcites. References: [1] Goldstein et al. (1984) Earth and Planetary Science Letter 70, 221-236. [2] Weldeab et al. (2011) Geophysical Research Letter 38, pp. 5

Persistently strong Indonesian Throughflow during marine isotope stage 3: evidence from radiogenic isotopes

Highlights: • First ITF reconstruction combining detrital and authigenic isotope signatures. • At the core site signatures of surface water cannot be separated from bottom water. • Intermediate to deep waters remain unchanged in western Timor Sea during MIS3. • Clay-size fraction of the sediment indicates a persistently strong ITF during MIS3. Abstract: The Indonesian Throughflow (ITF) connects the western Pacific Ocean with the eastern Indian Ocean, thus forming one of the major near surface current systems of the global thermohaline circulation. The intensity of the ITF has been found to be sensitive to changes in global ocean circulation, fluctuations in sea level, as well as to the prevailing monsoonal conditions of the Indonesian Archipelago and NW Australia. This study presents the first reconstruction of ITF dynamics combining radiogenic isotope compositions of neodymium (Nd), strontium (Sr), and lead (Pb) of the clay-size detrital fraction to investigate changes in sediment provenance, and paleo seawater Nd signatures extracted from the planktonic foraminifera and authigenic Fe–Mn oxyhydroxide coatings of the marine sediments focussing on marine isotope stage 3 (MIS3). Sediment core MD01-2378 was recovered within the framework of the International Marine Global Change Study (IMAGES) and is located in the area of the ITF outflow in the western Timor Sea (Scott Plateau, 13° 04.95′ S and 121° 47.27′ E, 1783 m water depth). In order to produce reliable seawater signatures, several extraction methods were tested against each other. The results of the study show that at this core location the extraction of surface water Nd isotope compositions from planktonic foraminifera is complicated by incomplete removal of contributions from Fe–Mn oxyhydroxides carrying ambient bottom water signatures. The bottom water Nd isotope signatures reliably obtained from the sediment coatings (average εNd = −5.0) document an essentially invariable water mass composition similar to today throughout the entire MIS3. The radiogenic Nd, Sr, and Pb isotope records of the clay-sized detrital fraction suggest that the Indonesian Archipelago rather than NW Australia was the main particle source at the location of core MD01-2378, and thus indicating a persistently strong ITF during MIS3. Furthermore, the variations of the detrital radiogenic isotopes are shown to be more sensitive to changes in circulation and document a somewhat enhanced ITF intensity during the early part of MIS3 until 47.4 ka compared with the remaining MIS3

The provenance of Cretaceous to Quaternary sediments in the Tarfaya basin, SW Morocco: Evidence from trace element geochemistry and radiogenic Nd–Sr isotopes

We present trace element compositions, rare earth elements (REEs) and radiogenic Nd–Sr isotope analyses of Cretaceous to recent sediments of the Tarfaya basin, SW Morocco, in order to identify tectonic setting, source rocks composition and sediments provenance. The results suggest that the sediments originate from heterogeneous source areas of the Reguibat Shield and the Mauritanides (West African Craton), as well as the western Anti-Atlas, which probably form the basement in this area. For interpreting the analyzed trace element results, we assume that elemental ratios such as La/Sc, Th/Sc, Cr/Th, Th/Co, La/Co and Eu/Eu∗ in the detrital silicate fraction of the sedimentary rocks behaved as a closed system during transport and cementation, which is justified by the consistency of all obtained results. The La/Y-Sc/Cr binary and La–Th–Sc ternary relationships suggest that the Tarfaya basin sediments were deposited in a passive margin setting. The trace element ratios of La/Sc, Th/Sc, Cr/Th and Th/Co indicate a felsic source. Moreover, chondrite-normalized REE patterns with light rare earth elements (LREE) enrichment, a flat heavy rare earth elements (HREE) and negative Eu anomalies can also be attributed to a felsic source for the Tarfaya basin sediments. The Nd isotope model ages (TDM = 2.0–2.2 Ga) of the Early Cretaceous sediments suggest that sediments were derived from the Eburnean terrain (Reguibat Shield). On the other hand, Late Cretaceous to Miocene–-Pliocene sediments show younger model ages (TDM = 1.8 Ga, on average) indicating an origin from both the Reguibat Shield and the western Anti-Atlas. In contrast, the southernmost studied Sebkha Aridal section (Oligocene to Miocene–Pliocene) yields older provenance ages (TDM = 2.5–2.6 Ga) indicating that these sediments were dominantly derived from the Archean terrain of the Reguibat Shield

Assessment of seawater Nd isotope signatures extracted from foraminiferal shells and authigenic phases of Gulf of Guinea sediments

The radiogenic neodymium (Nd) isotope composition of foraminiferal shells provides a powerful archive to investigate past changes in sources and mixing of water masses. However, seawater Nd isotope ratios extracted from foraminiferal shells can be biased by contaminant phases such as organic matter, silicates, or ferromanganese coatings, the removal of which requires rigorous multiple step cleaning of the samples. Here we investigate the efficiency of Flow Through and batch cleaning methods to extract seawater Nd isotope compositions from planktonic foraminifera in a shelf setting in the Gulf of Guinea that is strongly influenced by riverine sediment inputs. Nd isotope analyses of reductively and oxidatively cleaned mono-specific planktonic foraminiferal samples and reductively cleaned mixed benthic foraminifera were complemented by analyses of non-reductively cleaned mono-specific planktonic foraminiferal samples, Fe–Mn coatings of de-carbonated bulk sediment leachates, and the residual detrital fraction of the same sediment. Al/Ca and Mn/Ca ratios of fully cleaned foraminiferal samples reveal indistinguishable levels of cleaning efficiency between the batch and the Flow Through methods and the Nd isotope compositions obtained from application of both methods are identical within error. Furthermore, non-reductively cleaned foraminiferal samples have the same Nd isotope composition as reductively cleaned foraminifera at our study sites. Close to the Niger River mouth the Nd isotope composition of the foraminifera agree with the seawater Nd isotope composition of nearby stations. Based on the combined extracted Nd isotope signatures and element to calcium ratios, as well as rare earth element distribution patterns, we infer that the planktonic foraminiferal Nd isotope signatures reflect bottom water/pore water signatures. The isotopic composition of the bulk de-carbonated sediment leachates (Fe–Mn coatings) differs significantly from the foraminiferal data at this site and probably reflects particles that acquired their ferromanganese/pre-formed pre-formed/ferromanganese coatings in nearby rivers. Therefore, in such river influenced shelf settings foraminiferal shells should be used to obtain unbiased bottom seawater signatures