ALBERT

Description: The potential of mining seafloor massive sulfide deposits for metals such as Cu, Zn, and Au is currently debated. One key challenge is to predict where the largest deposits worth mining might form, which in turn requires understanding the pattern of subseafloor hydrothermal mass and energy transport. Numerical models of heat and fluid flow are applied to illustrate the important role of fault zone properties (permeability and width) in controlling mass accumulation at hydrothermal vents at slow spreading ridges. We combine modeled mass-flow rates, vent temperatures, and vent field dimensions with the known fluid chemistry at the fault-controlled Logatchev 1 hydrothermal field of the Mid-Atlantic Ridge. We predict that the 135 kilotons of SMS at this site (estimated by other studies) can have accumulated with a minimum depositional efficiency of 5% in the known duration of hydrothermal venting (58,200 year age of the deposit). In general, the most productive faults must provide an efficient fluid pathway while at the same time limit cooling due to mixing with entrained cold seawater. This balance is best met by faults that are just wide and permeable enough to control a hydrothermal plume rising through the oceanic crust. Model runs with increased basal heat input, mimicking a heat flow contribution from along-axis, lead to higher mass fluxes and vent temperatures, capable of significantly higher SMS accumulation rates. Nonsteady state conditions, such as the influence of a cooling magmatic intrusion beneath the fault zone, also can temporarily increase the mass flux while sustaining high vent temperatures. © 2017. American Geophysical Union. All Rights Reserved.

Description: We reanalyze existing paleodata of global mean surface temperature ΔTg and radiative forcing ΔR of CO2 and land ice albedo for the last 800,000 years to show that a state‐dependency in paleoclimate sensitivity S, as previously suggested, is only found if ΔTg is based on reconstructions, and not when ΔTg is based on model simulations. Furthermore, during times of decreasing obliquity (periods of land ice sheet growth and sea level fall) the multimillennial component of reconstructed ΔTg diverges from CO2, while in simulations both variables vary more synchronously, suggesting that the differences during these times are due to relatively low rates of simulated land ice growth and associated cooling. To produce a reconstruction‐based extrapolation of S for the future, we exclude intervals with strong ΔTg‐CO2 divergence and find that S is less state‐dependent, or even constant state‐independent), yielding a mean equilibrium warming of 2–4 K for a doubling of CO2.

Description: Evidence from the joint interpretation of proxy data as well as geodynamical and biogeochemical modeling results point to complex interactions between sea level drawdown, volcanic degassing, and atmospheric CO2 that hampered the climate system’s decent into the last ice age. Ice core data shows that atmospheric CO2 dropped abruptly into glacial Marine Isotope Stage (MIS) 4 at ~71 ka, while Antarctic temperatures display a more gradual decline between ~85 ka to ~71 ka across the MIS 5/4 transition. Based on 2D and 3D geodynamical simulations, we show that a ~60-100 m sea level drop associated with the MIS 5/4 transition led to a significant increase in magma and possibly CO2 flux at mid-ocean ridges (MOR) and oceanic hotspot volcanoes. The MOR signal is assessed with 2D thermomechanical models that account for mantle melting and resolve the flux of incompatible carbon dioxide. These models have been run at different spreading rates and integrated with the global distribution of opening rates to compute global variations in magma and CO2 flux across the MIS 5/4 transition. 3D plume models have been used to quantify the impact of a dropping sea level on oceanic hotspot melting and CO2 release. Here a wide range of simulations with differing plume fluxes, lithospheric thicknesses as well as speeds, and plume excess temperatures have been integrated with data from ~40 hotspots in order to compute a global signal. Biogeochemical carbon cycle modeling shows that the predicted increase in volcanic emissions is likely to have raised atmospheric CO2 by up to 15 ppmv, sufficient to explain the bulk of the decoupling between temperature and atmospheric CO2 during the global change to pronounced glacial conditions across the MIS 5/4 transition.

Description: Increasing evidences point to a more important role of volcanic CO2 outgassing in the carbon cycle than previously thought. Here we present two examples, where data or models indicate that only volcanic CO2 outgassing might explain some observed phenomena. (1) The paleo data show that atmospheric CO2 and Antarctic temperature changes are surprisingly synchronous on both millennial and orbital time scale, although there are some still unexplained exceptions. Here we show that the decoupling of temperature and CO2 around the transition into full glacial conditions around the MIS 5/4 boundary (~75kyr BP) might have been caused by the volcanic CO2 degassing, that itself was triggered by the sea level fall of 60-100m within ~10kyr. An additional volcanic CO2 release from mid ocean ridges and hotspots calculated with a state-of-the- art 3D geodynamical model to ~500 to 900 GtCO2 might explain the bulk of the ~18 ppm CO2 anomaly, that is associated with this decoupling of CO2 and temperature on orbital time scales. (2) Radiocarbon (14C) is widely used to detect the carbon that has been transferred from the atmosphere to the deep ocean during the LGM. New 14C data from a depth transect indicate that this carbon might be found at mid water depths (~3 km) in the South Pacific. However, the maximum observed anomaly in deep ocean Δ14C to the atmosphere of -1000permil can only be explained if a realistic increase in reservoir age and a hydrothermal influx of 14C-free CO2 from mid ocean ridges are considered together.

Description: Following Milankovitch's theory the incoming insolation or summer energy at 65°N is typically analysed to predict the waxing or waning of land ice. We here use a model-based deconvolution of the LR04 benthic-d18O stack into land ice distribution (de Boer et al., 2014, Köhler et al., 2015) to verify if the latitudinal focal point of land ice dynamics has changed over the last 2 Myr or whether this choice of 65°N in orbital data is indeed well justified. We find that the 5°-latitudinal band which contributes most to land ice albedo radiative forcing (ΔR_[LI]) is 70-75°N between 2.0-1.5 Myr, which is then until 1.0 Myr gradually substituted by 65-70°N. During the last 1 Myr both 60-65°N and 65-70°N dominate ΔR_[LI] and contribute approximately the same amount, while the relative importance of 70-75°N is shrinking. Our analyses illustrates that the choice of 65°N seems for the last 1 Myr to be well justified, while for earlier parts of the last 2 Myr the dominant land ice changes seems to happen up to 10° further to the north. Focusing on the last 800 kyr (the time for which precise data on atmospheric CO2 concentration exists) we furthermore find that the multi-millennial land ice growth and proxy-based reconstruction of global cooling (= the glaciation) appear synchronously to each other and to decreasing obliquity, but diverge from CO2. This suggests that the global cooling associated with Earth's way into an ice age as deduced in the reconstructions has to be mainly caused by the land ice albedo feedback, and is not dominated by the CO2 greenhouse forcing. One way of perceiving this CO2-glaciation divergence in reconstructions is that the reduced incoming insolation at high latitudes causes land ice growth and cooling, while there is a coexisting process that keeps CO2 at a relatively constant level. Solid Earth modeling experiments have indicated that falling sea level might lead to enhanced magma and CO2 production at mid-ocean ridges. Hasenclever et al. (2017) suggested that the combination of marine volcanism at mid-ocean ridges and at hot spot island volcanoes might react to decreasing sea level and be a potential cause for this CO2-glaciation divergence. This CO2-glaciation divergence needs to be considered, when using paleo data to quantify paleoclimate sensitivity: periods with diverging CO2 and global temperature change should be filtered out when approximating the relationship between global temperature rise and CO2 concentrations (Köhler et al., 2018). References: de Boer et al. (2014). https://doi.org/10.1038/ncomms3999. Köhler et al. (2015). https://doi.org/10.5194/cp-11-1801-2015. Hasenclever et al. (2017). https://doi.org/10.1038/ncomms15867. Köhler et al. (2018). https://doi.org/10.1029/2018GL077717.

Description: The potential of mining Seafloor Massive Sulfide deposits for metals such as Cu, Zn, and Au is currently debated. One key challenge is to predict where the largest deposits worth mining might form, which in turn requires understanding the pattern of sub-seafloor hydrothermal mass and energy transport. Numerical models of heat and fluid flow are applied to illustrate the important role of fault zone properties (permeability and width) in controlling mass accumulation at hydrothermal vents at slow-spreading ridges. We combine modeled mass-flow rates, vent temperatures and vent field dimensions with the known fluid chemistry at the fault-controlled Logatchev 1 hydrothermal field of the Mid-Atlantic Ridge. We predict that the 135 kilotons of SMS at this site (estimated by other studies) can have accumulated with a minimum depositional efficiency of 5% in the known duration of hydrothermal venting (58,200 year age of the deposit). In general, the most productive faults must provide an efficient fluid pathway while at the same time limit cooling due to mixing with entrained cold seawater. This balance is best met by faults that are just wide and permeable enough to control a hydrothermal plume rising through the oceanic crust. Model runs with increased basal heat input, mimicking a heat flow contribution from along-axis, lead to higher mass fluxes and vent temperatures, capable of significantly higher SMS accumulation rates. Non-steady state conditions, such as the influence of a cooling magmatic intrusion beneath the fault zone, also can temporarily increase the mass flux while sustaining high vent temperatures.

Description: High-temperature (〉300 °C) off-axis hydrothermal systems found along the slow-spreading Mid-Atlantic Ridge are apparently consistently located at outcropping fault zones. While preferential flow of hot fluids along highly permeable, fractured rocks seems intuitive, such efficient flow inevitably leads to the entrainment of cold ambient seawater. The temperature drop this should cause is difficult to reconcile with the observed high-temperature black smoker activity and formation of associated massive sulfide ore deposits. Here we combine newly acquired seismological data from the high-temperature, off-axis Logatchev 1 hydrothermal field (LHF1) with numerical modeling of hydrothermal flow to solve this apparent contradiction. The data show intense off-axis seismicity with focal mechanisms suggesting a fault zone dipping from LHF1 toward the ridge axis. Our simulations predict high-temperature venting at LHF1 only for a limited range of fault widths and permeability contrasts, expressed as the fault’s relative transmissibility (the product of the two parameters). The relative transmissibility must be sufficient to “capture” a rising hydrothermal plume and redirect it toward LHF1 but low enough to prevent extensive mixing with ambient cold fluids. Furthermore, the temperature drop associated with any high permeability zone in heterogeneous crust may explain why a significant part of hydrothermal discharge along slow-spreading ridges occurs at low temperatures.