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The East African Rift System (EARS) is a key location for studying Plio-Pleistocene paleoclimate and hominin inhabitance. The region experienced profound reorganization during this interval as a response to volcanism, tectonics and climate change, and arguably detailed spatiotemporally coherent climate datasets could provide evidence of causal links between geologic change and hominin evolution. However, continued tectonism, erosion, burial and volcanism obscures much of this information. Despite its rich fossil record, the Turkana basin in the northern Kenya Rift is no exception. It has been hypothesized that Lake Turkana and paleo-Lake Suguta to its south formed one 530-650 km long mega-lake before 221 ka ago, and was a major barrier for E-W dispersal of hominids and other terrestrial fauna. Here we present new information on basin development based on paleolandscape modeling and 87Sr/86Sr analysis on microfossils of newly discovered paleo-lake sequences in the Suguta Valley, permitting reconstruction of volcano-tectonic processes 900-700 ka ago. Contrary to previous assumptions, results suggest that two to three lakes separated by tectono-volcanic barriers formed instead of one mega-lake. These results have implications for previously formulated hypotheses about mega-lakes preventing W-E migration and exchange and suggest that during the early Middle Pleistocene E-W migrations were possible.

Continuous lake sediment archives integrate valuable information of geodynamic transformations, climatic fluctuations and anthropogenic environmental forcing through time. In many parts of the world, such as sub-Saharan Africa, lake ecosystems are important pillars of biodiversity and wildlife preservation and evolution, as well as political and economic stability, especially with regard to the rapid population growth and increasing food and water demand.Located in the central part of the rift valley region, lake Naivasha is the second largest freshwater lake in Kenya, covers a catchment area of ca. 3400 km2 and is considered a “wetland of international importance” (RAMSAR convention, 2011). Previous studies and real-time observations documented a rapid intensification of agricultural activities ranging from subsidy economy (upper catchment) to industrial-sized horticulture practices (lower catchment) from the second half of the 20th century towards the present. These were suggested to have significantly influenced the drainage systems of the catchment and hydrochemistry of the lake, with potentially negative effects on the entire ecosystem. In addition, potential anthropogenic metal influx from other modern, diffuse sources (such as fossil fuel combustion) due to the increasing anthropogenic density and activities in the immediate vicinity of the lake remain poorly constrained.We analysed major- and trace elements and Pb isotope compositions of lake sediments covering the past ca. 150 years, as well as the surrounding lithologies in order to reconstruct the pathway(s) and source(s) of elemental influx and accumulation into the lake. The characterization of the geological background in this tectonically and volcanologically active region was primarily set on the northern part of the catchment where, the two main lake-feeding rivers Malewa and Gilgil discharge into the lake. Element correlation indices point to i) a strong influence of the local geological background and, ii) a relatively stable catchment for this time-period as seen from sub-parallel REE+Y patterns along the monolith. Lead isotope compositions, on the other hand, show more radiogenic values in the sediment deposited before the 1900’s (206Pb/204Pb: 19.502 – 19.546) and a significant shift towards less radiogenic isotopic ratios from the second half of the 20th century (206Pb/204Pb: 19.228 – 19.304), which persists towards the top of the core. We combine our extended geochemical data with geospatial projections of the land use to build a time-integrated cause-and-effect assessment of metals into lake Naivasha and disentangle the cause for the change in the Pb isotope composition.

Ultra-trace ( HREE-enriched REE + Y patterns, small natural positive Y and Gd anomalies, and negative Eu and Ce anomalies. These REE + Y features are coherent downstream in the OR apart from amplification of Eu and Ce anomalies during REE removal/dilution. The OR samples capture a downstream decrease in sparingly soluble HFSE (Th, Nb, Ta, Zr, Hf), presumably related to their colloid-particulate removal from the dissolved load, accompanied by crustal Zr/Hf (32.5 ± 5.1) and supercrustal Nb/Ta (25.1 ± 7.7) ratios. Subcrustal Th/U (0.17–0.96) and supercrustal Mo/W (12.0–74.5) ratios in all ORB waters indicate preferential release and aqueous solubility of U > Th and Mo > W, with the latter attributed primarily to preferential W adsorption on soil or upstream aquatic (oxy)(hydr)oxide surfaces.

Significant Mo mobility and isotope (δ98/95Mo) fractionation is induced during prograde metamorphism at present-day subduction zones. Depending on the redox conditions during early subduction and accompanied slab dehydration, isotopically heavy Mo is released towards the overlying mantle wedge, leaving behind a depleted, and isotopically light subducted slab. This isotopically light Mo signature has been detected in slab-melt influenced volcanic rocks and potentially will be traceable in ocean-island basalts, if their geochemical signatures are affected by previously subducted lithologies (i.e. slab and overlying sediments). Thus, the isotope composition of mantle plume-influenced volcanic rocks might reveal the nature of subducted and re-incorporated lithologies and possibly redox conditions during subduction.In this study, we present new Mo isotope data for South-Mid Atlantic Ridge basalts that partly interacted with the enriched Discovery and Shona mantle plumes. Isotopically heavier Mo isotope ratios (δ98/95Mo > ambient depleted mantle) are observed in samples tapping a more enriched mantle source. Furthermore, δ98/95Mo correlates with radiogenic isotopes (Sr, Nd, Hf) indicating recycling of a Proterozoic sedimentary components with a Mo isotopic composition that was not modified during and before subduction by Mo mobility under oxidising conditions. Rather, the new Mo isotope data supports and expands on previous stable Se and S isotope evidence that suggests the incorporation of subducted anoxic Proterozoic deep-sea sediments into the mantle of the South-Mid Atlantic Ridge basalts.

Our understanding regarding the behaviour of the fluid mobile element Zn at destructive plate margins is limited. In particular the fractionation mechanisms and input-output mass-balance remains to be investigated due to implications for the spatio-temporal cycling of this vital and socio-economically relevant element. In this study, we investigate the Zn isotope systematics of subduction input provided by IODP samples from the SW Pacific in comparison to lavas from the central Tonga arc, addressed as a worldwide endmember in terms of pre-subduction mantle wedge depletion. With an improved analytical precision, we report subtle, yet resolvable Zn isotope variations between the central Tongan islands, with an overall statistically relevant variation of 0.05‰ (at ±0.014‰ 2SD). The signatures are all > 0.1‰ lighter than the subduction input at this site, suggesting a fractionation process during subduction. After careful extraction of the isotopic effect caused by mantle melting processes (using DMM δ66/64Zn JMC-Lyon provided by Sossi et al. (2018) and Wang et al. (2017) and melt extraction indices such as Sm/La, Zr/Nb, and Zn/La), a pronounced negative correlation is observed between the Zn isotopic composition of the lavas and key fluid indicators such as Ba/Th and Ce/Pb. Together with predictions from ab initio calculations and mixing models performed between Indian DMM and Rayleigh dehydration of the subducting slab, we attribute the remaining, subtle Zn isotope variations to additions by Cl-rich fluids to the individual mantle wedges. A maximum of 5% chlorine-fluid contribution is suggested for the magmatic source of Tofua, whereas smaller proportions are estimated for Kao, Late and Ata. Overall, this study sheds new light on Zn fractionation mechanisms in sediment-poor subduction zones. Implications for the long-term Zn recycling will be addressed in this presentation.References:Sossi, P.A., Nebel, O., O’Neill, H.S.C., Moynier, F., 2018. Zinc isotope composition of the Earth and its behaviour during planetary accretion. Chemical Geology 447, 73-84.Wang, Z.-Z., Liu, S.-A., Liu, J., Huang, J., Xiao, Y., Chu, Z.-Y., Zhao, X.-M., Tang, L., 2017. Zinc isotope fractionation during mantle melting and constraints on the composition of Earth’s upper mantle. Geochimica et Cosmochimica Acta 198, 151- 167.

Unravelling inputs of multiple air pollution sources and reconstructing their historic contribution can be a difficult task. Here, new trace metal concentrations and Pb isotope data were combined for a radionuclide (Pb-210-Am-241) dated peat core from the Liffey Head bog (LHB) in eastern Ireland in order to reconstruct how different sources contributed to the atmospheric pollution over the past century. Highest enrichments in the heavy metals Pb, Cu, Ag, Sn, and Sb, together with a Pb isotope composition (Pb-206/Pb-204: 18.351 +/- 0.013; Pb-206/Pb-207: 1.174 +/- 0.012) close to that of the Wicklow mineralisation demonstrates significant aerial influx of heavy metals from local mining and smelting activities during the 19th century until ca. 1940s. A dramatic compositional shift defined by elevated Co, Cr, Ni, Mo, Zn, and V enrichments and a sharp transition towards unradiogenic 206Pb values (Pb-206/Pb-204: 18.271 +/- 0.013-17.678 +/- 0.006; Pb-206/Pb-207: 1.170 +/- 0.012-1.135 +/- 0.007) is documented from the 1940s until ca. 2000. These are attributed to the atmospheric impact of fossil fuels and especially leaded petrol, modelled to have contributed between 6 and 78% to the total Pb pollution at this site. The subsequent turn to a more radiogenic Pb isotope signature since 2000 in Ireland is clearly documented in the investigated archive (Pb-206/Pb-204: 17.930 +/- 0.006; Pb-206/Pb-207: 1.148 +/- 0.007) and reflects the abolishment of leaded petrol. However, there remains a persisting and even increasing pollution in Ni, Mo, Cu, and especially Zn, collectively originating from countrywide use of fossil fuels (peat, coal, heating oil, and unleaded vehicle fuels) for domestic and industrial purposes. This illustrates the continued anthropogenic influence on important natural archives such as bogs in Ireland despite the phase-out of leaded petrol.

Molybdenum isotopes have emerged as novel tracers for high-temperature igneous and metamorphic processes. The debate remains to what extent different subducted slab lithologies, such as oceanic crust and marine sediments, contribute to the Mo isotope signature of arc magmas and, hence, exert different controls on the terrestrial Mo cycle. Here we investigate Mo isotope systematics from input to output at the Tongan subduction zone: Arc lavas from different Tongan islands, pelagic sediments and altered oceanic crust (AOC) samples from DSDP site 595/596 on the subducting Pacific plate. For complementary insights into the fate of Mo and its isotopic signatures during prograde subduction metamorphism, we also present data of metasediments and variably altered AOC-type eclogites from the Zermatt-Saas ophiolite, Switzerland and Italy, and the Schistes Lustres Complex in Alpine Corsica. Manganese oxide-rich pelagic sediments from DSDP site 595/596 show variable, depth-dependent Mo/Mn ratios and Mo isotope compositions controlled by diagenetic reactions. As subducted equivalents, Mn-rich eclogitic metapelites display lower Mo contents and δ98/95Mo ratios compared to their non-subducted protolith. This indicates prominent loss of Mo along with isotope fractionation during early subduction metamorphism. In comparison to unaltered MORB, low temperature seafloor alteration has shifted Mo/Ce and δ98/95Mo in studied AOC samples towards lower ratios, in the range of most mafic eclogites published so far. However, some mafic eclogites show even lower Mo/Ce and δ98/95Mo ratios compared to AOC, likely due to fluid-related Mo loss upon early subduction and preferential incorporation of light Mo into residual rutile. Our data document a prominent loss of isotopically heavy Mo before and upon early subduction metamorphism at shallow depths in the forearc region. Moreover, when prograde rutile crystallizes at ~30 km depth, it fixes the largest fraction of Mo in the subducting material. This creates an “arc Mo-conundrum” as devolatization of the slab at subarc depths is not able to account for the fluid-mobile Mo source responsible for observed higher Mo/Ce and δ98/95Mo in Tongan arc lavas compared to the mantle. As an alternative scenario, Mo mobilization by slab-derived aqueous fluids during the early stages of subduction into the forearc mantle produces serpentinites enriched in Mo with a possible heavy Mo isotopic signature. Mechanical transport and devolatization of forearc serpentinites at subarc regions is a plausible alternative recycling process accounting for the observed Mo systematics in Tongan arc lavas. This is supported by positive covariations of Mo/Ce and δ98/95Mo with elements such as As, Sb, and Cs, which are thought to be mostly released from the subducting material during early stages of subduction. We propose that a multi-stage recycling of metasomatized forearc mantle can be an important process in recycling of Mo and possibly other elements.

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Atmospherically-fed Earth surface archives such as ombrotrophic peatlands, lake sediments, and ice consistently show an upward increase in Zn concentrations of hitherto unclear origin. Here, we present a combined stable Zn isotope and trace element (Zn, Cd, Ni, Cu, Cr, V, Ta, Pb) dataset for a historically polluted, near-urban bog (Liffey Head) from the east coast of Ireland. This peat record is compared to an archive from a rural site at the west coast of Ireland (Brackloon Wood). Both archives show a clear near-surface increase in Zn deposition, accompanied by periodic deposition in Cr, Ni, Mo, and V suggesting a co-genetic origin of these elements. In the Liffey Head site, biologic upward distillation of nutrients can be excluded as the origin of the elemental enrichments. The differences in the excess metal ratios between the two sites (e.g., Zn/Cd of 426–1564, east, and 77–106, west) are attributed to a higher contribution from traffic emissions (diesel, petrol) and oil-burning at the near-urban site, and dominant atmospheric influence from solid fossil fuel combustion emissions (e.g., mixed fuel, coal and wood) at the rural site. The Zn isotope composition in the historically-polluted Liffey Head bog evolved from δ66/64ZnJMC-Lyon values of 0.72 ± 0.03‰ in the peat accumulated during the 19th century to lighter ratios (0.18 ± 0.03‰) towards the top of the monolith (i.e., recent). Zinc-isotope ratios are positively correlated with excess metal/Cd ratios and also with 206Pb/207Pb, collectively fingerprinting the gradual change from a mining-dominated to a traffic-dominated atmospheric pollution at the east coast over the past century. A prevalent input, interpreted to represent combustion emissions from diesel engines, is observed for the past 15 years. Combined with trace elements and radiogenic Pb isotopes, the information obtained with the Zn isotope systematics adds towards an in-depth characterisation of the pollution signals.

Zinc (Zn) is a micronutrient for organisms and essential for plant growth, therefore knowledge of its elemental cycling in the surface environment is important regarding wider aspects of human nutrition and health. To explore the nature of Zn cycling, we compared its weathering behaviour in a sub-recent regolith versus an ancient laterite profile of the Deccan Traps, India - an area of known soil Zn deficiency. We demonstrate that progressive breakdown of primary minerals and the associated formation of phyllosilicates and iron oxides leads to a depletion in Zn, ultimately resulting in a loss of 80% in lateritic residues. This residue is mainly composed of resistant iron oxides and hydroxides ultimately delivering insufficient amounts of bio-available Zn. Moreover, (sub)-tropical weathering in regions experiencing extended tectonic quiescence (e.g., cratons) further enhance the development of old and deep soil profiles that become deficient in Zn. This situation is clearly revealed by the spatial correlation of the global distribution of laterites, cratons (Africa, India, South America and Australia) and known regions of Zn deficient soils that result in health problems for humans whose diet is derived from such land. We also investigate whether this elemental depletion of Zn is accompanied by isotope fractionation. In the saprolitic horizons of both weathering profiles, compositions of δ66ZnJMC-Lyon lie within the "crustal average" of +0.27±0.07‰ δ66ZnJMC-Lyon. By contrast, soil horizons enriched in secondary oxides show lighter isotope compositions. The isotopic signature of Zn (Δ66Znsample-protolith up to ~ -0.65‰) during the formation of the ferruginous-lateritic weathering profile likely resulted from a combination of biotically- and kinetically-controlled sorption reactions on Fe-oxyhydroxides. Our findings suggest that oxide rich soil types/horizons in (sub)-tropical regions likely exert a control on riverine Zn isotope compositions such that these become heavier than the crustal average. This isotopic behaviour invites a broader study of global soils to test whether light isotope composition alone could serve as an indicator for reduced bioavailability of Zn.