Description: After more than a decade of multidisciplinary studies of the Central American subduction zone mainly in the framework of two large research programmes, the US MARGINS program and the German Collaborative Research Center SFB 574, we here review and interpret the data pertinent to quantify the cycling of mineral-bound volatiles (H2O, CO2, Cl, S) through this subduction system. For input-flux calculations, we divide the Middle America Trench into four segments differing in convergence rate and slab lithological profiles, use the latest evidence for mantle serpentinization of the Cocos slab approaching the trench, and for the first time explicitly include subduction erosion of forearc basement. Resulting input fluxes are 40–62 (53) Tg/Ma/m H2O, 7.8–11.4 (9.3) Tg/Ma/m CO2, 1.3–1.9 (1.6) Tg/Ma/m Cl, and 1.3–2.1 (1.6) Tg/Ma/m S (bracketed are mean values for entire trench length). Output by cold seeps on the forearc amounts to 0.625–1.25 Tg/Ma/m H2O partly derived from the slab sediments as determined by geochemical analyses of fluids and carbonates. The major volatile output occurs at the Central American volcanic arc that is divided into ten arc segments by dextral strike-slip tectonics. Based on volcanic edifice and widespread tephra volumes as well as calculated parental magma masses needed to form observed evolved compositions, we determine long-term (105 years) average magma and K2O fluxes for each of the ten segments as 32–242 (106) Tg/Ma/m magma and 0.28–2.91 (1.38) Tg/Ma/m K2O (bracketed are mean values for entire Central American volcanic arc length). Volatile/K2O concentration ratios derived from melt inclusion analyses and petrologic modelling then allow to calculate volatile fluxes as 1.02–14.3 (6.2) Tg/Ma/m H2O, 0.02–0.45 (0.17) Tg/Ma/m CO2, and 0.07–0.34 (0.22) Tg/Ma/m Cl. The same approach yields long-term sulfur fluxes of 0.12–1.08 (0.54) Tg/Ma/m while present-day open-vent SO2-flux monitoring yields 0.06–2.37 (0.83) Tg/Ma/m S. Input–output comparisons show that the arc water fluxes only account for up to 40 % of the input even if we include an “invisible” plutonic component constrained by crustal growth. With 20–30 % of the H2O input transferred into the deeper mantle as suggested by petrologic modeling, there remains a deficiency of, say, 30–40 % in the water budget. At least some of this water is transferred into two upper-plate regions of low seismic velocity and electrical resistivity whose sizes vary along arc: one region widely envelopes the melt ascent paths from slab top to arc and the other extends obliquely from the slab below the forearc to below the arc. Whether these reservoirs are transient or steady remains unknown.

Description: We investigated the halogen (Cl, F, Br, and I) chemistry of serpentinites that record progressive dehydration during subduction from shallow oceanic environments via increased pressure and temperature conditions to complete breakdown of antigorite. The aim is to evaluate the relevance of serpentinites for halogen recycling in subduction zones and for deep mantle recharge of these elements. The halogen compositions of the analyzed samples indicate input from seawater and sedimentary sources during initial serpentinization of either subducting lithospheric mantle during slab bending or forearc mantle by uprising slab fluids. During the first dehydration stage (antigorite + brucite → olivine + H2O), fluids with high Br/Cl and I/Cl ratios are released resulting in residual serpentinites with lower Br/Cl and I/Cl ratios. Veins associated with this event and with the final antigorite breakdown (antigorite → olivine + orthopyroxene + H2O) show higher halogen ratios compared to their adjacent wall rocks, and they are similar to those found in arc volcanoes (F/Cl and I/Cl between ca. 0.083–1.5, and ca. 0.00038–0.0013, respectively). All measured deserpentinization samples show a narrow range in δ37Cl values (between − 0.42‰ and + 0.92‰) overlapping the δ37Cl values of seafloor serpentinites and confirming that no significant Cl isotope fractionation occurs during subduction dehydration of serpentinites. Our findings document the conservative behavior of halogens during subduction. Mass balance constraints reveal that serpentinites strongly control the halogen chemistry of deep subduction zone fluids and that descent of rock residues after deserpentinization strongly affects the halogen budget of the mantle.

Description: Individual migration behaviour during the juvenile and adult life phase of the anadromous twaite shad Alosa fallax in the Elbe estuary was examined using otolith Sr:Ca and Ba:Ca profiles. Between hatching and the end of the first year of life, juveniles showed two migration patterns. Pattern one exhibited a single downstream migration from fresh water to the sea with no return into fresh water. In contrast, pattern two showed a first migration into the sea, then a return into fresh water and, finally, a second downstream migration into marine water. This first report of migration plasticity for A. fallax points to different exposure times to estuarine threats depending on the migration strategy. In adults, high Sr:Ca and low Ba:Ca in the majority of individuals confirmed prior reports of a primarily marine habitat use. Patterns reflecting spawning migrations were rarely observed on otoliths, possibly due to the short duration of visits to fresh water.

Description: Volatile induced melting processes at mantle depth of convergent margins are triggered by sequential devolatilizing of the subducting oceanic slab. We show that stable chlorine isotopes (37Cl, 35Cl, expressed as 37Cl) can be used to trace each volatile source involved in melt generation and arc volcanism: During dewatering processes in subduction zones, chlorine is strongly partitioned into the aqueous fluid phase, while preserving the distinct ᵟ37Cl values of its volatile sources; Chlorine-isotope fractionation is marginal at high (magmatic) temperatures or during subduction metamorphism and is apparently limited to low temperature processes. Due to limited range of ᵟ37Cl-values in silicate rocks (about ± 2‰) and the relatively imprecise analytical data in earlier studies, few convincing geochemical trends for stable chlorine isotopes have been published so far. To archive the required analytical precision, we developed an alternative and more exact (~0.05‰ 2#) method for ᵟ37Cl determination via LA-MC-ICP-MS. Here we present high-precision ᵟ37Cl data of basaltic to rhyolitic volcanic glasses collected about 700 km along the Central America Volcanic Arc (CAVA). The observed systematic isotopic variations spans a relatively small range in !37Cl values of -1.15 to +0.96‰ and represent systematic changes in volatile sources and depth-dependent zones of fluid release. Our results confirm models for continuous slab dewatering and melt generation and highlight the conservative behavior of stable chlorine isotopes during recycling processes in subduction zones.

Description: The Nicaraguan volcanoes, which are part of the Central American Volcanic Arc (CAVA), have produced 13 highly explosive eruptions within the last 60 ka. All of these “Plinian” eruptions reached well into the stratosphere such that their released volatiles may have induced changes of atmospheric chemistry and climate. While previous research has focussed on the sulfur and chlorine emissions during such large eruptions, we here present measurements of the heavy halogens (bromine, iodine) by means of synchrotron-XRF microanalysis. Spot analyses of pre-eruptive glass inclusions trapped in minerals formed in magma reservoirs were compared with those in matrix glasses of the tephras, which represent the post-eruptive, degassed concentrations. The concentration difference between inclusion and matrix, multiplied by erupted magma mass determined by extensive field mapping, yields an estimate of the degassed mass of heavy halogens. Br and I are probably hundreds of times more effective in destroying ozone than Cl, and can accumulate in the stratosphere over significant time scales. The analysed deposits of 12 large eruptions have bromine and iodine contents in the inclusions of 5 to 27 ppm and 1 to 3 ppm, respectively. Br and I concentrations in matrix glasses are nearly constant at 5 ppm and 0.5 ppm, respectively. Analyses by mass spectrometry of pyrohydrolysed samples of the phenocryst-poor bulk glasses confirm the Br and I concentrations observed for matrix glasses. On average, masses of 50 kilotons of bromine and 27 kilotons of iodine have been released during single eruptions; for the 25 ka eruption of the Upper Apoyo Tephra, however, the released heavy halogen masses are about one order of magnitude larger, reflecting both the larger erupted magma mass and higher magmatic heavy halogen concentrations. Our new data on heavy halogens will be used as input for climate models to evaluate past –and possible future – atmospheric effects.

Description: Throughout the transition from the last Glacial to the current Interglacial, rising atmospheric CO2 levels were accompanied by declining atmospheric Δ14C values. A likely mechanism, influencing both components is the deglacial release of CO2, stored for millennia in the deep Ocean, to the atmosphere. Due to its long residence time within the oceans interior this CO2 rich water mass was considerably depleted in radiocarbon. Although a large number of studies address this topic, the extent, location and pathways of the glacial carbon pool are still subjects of an ongoing debate. As deep water masses are upwelled and new intermediate waters are formed around Antarctica, the Southern Ocean is a potential area for the deglacial release of stored CO2. Here we present radiocarbon and carbonate ion data from a transect of sediment cores off New Zealand that covers the major water masses in this area, from the AAIW down to the AABW. During the Glacial, our data locate a significantly 14C depleted pool in a water depth between 2000 and 4500 m. The combination of Δ14C and [CO32-] records provides new insights into the process of oceanic-atmospheric CO2 exchange in the Southern Ocean. In addition, our results yield new implications for contradicting Δ14C records from the Southern Ocean and lower latitudes.