[1] The basal regions of continental ice sheets are gaps in our current understanding of the Earth's biosphere and biogeochemical cycles. We draw on existing and new chemical data sets for subglacial meltwaters to provide the first comprehensive assessment of sub-ice sheet biogeochemical weathering. We show that size of the ice mass is a critical control on the balance of chemical weathering processes and that microbial activity is ubiquitous in driving dissolution. Carbonate dissolution fueled by sulfide oxidation and microbial CO 2 dominate beneath small valley glaciers. Prolonged meltwater residence times and greater isolation characteristic of ice sheets lead to the development of anoxia and enhanced silicate dissolution due to calcite saturation. We show that sub-ice sheet environments are highly geochemically reactive and should be considered in regional and global solute budgets. For example, calculated solute fluxes from Antarctica (72-130 t yr −1 ) are the same order of magnitude as those from some of the world's largest rivers and rates of chemical weathering (10-17 t km −2 yr −1 ) are high for the annual specific discharge (2.3-4.1 × 10 −3 m). Our model of chemical weathering dynamics provides important information on subglacial biodiversity and global biogeochemical cycles and may be used to design strategies for the first sampling of Antarctic Subglacial Lakes and other sub-ice sheet environments for the next decade.

ABSTRACT. To ascerta in whether the velocity orIce Stream B, '!\Test Antarctica, m ay be controlled by the stresses in its m argin al shear zones (the "Sn a ke" a nd the "Dragon" ), we undertook a determin atio n of the ma rgina l shea r stress in th e Dragon near C a mp Up B by using iee itself as a stress m e ter. The obser ved m arginal shea r stra in rate of 0.14 a-I is used to calc ul a te the margina l shea r stress from the now law of ice d etermined by cr eep tests on ice cores from a depth o f 300 m in the Dragon, obtain ed by using a hot-wa ter icecoring drill. Th e tes t-specimen o rientati on relative to the stress axes in th e tests is chosen on the basis o f c-axis fabrics so th at the test appli es hori zonta l shea r across \'ertic;al pl a nes pa rallel to th e margin . The res ulting margina l sh ear stress is (2.2 ± 0.3) x 10~ Pa . This impli es th a t 63-100 % of the ice stream's support aga inst gravita ti o na l loading comes from the margins a nd only 37-0 % from the base, so tha t th e margins pl ay a n important rol e in controlling the ice-stream motion. The marginal shear-stress va lue is twice that g ive n by the ice-stream model of Echelmeyer and others (1994) and th e co rres ponding stra in-rate enhancem ent factors differ greatly (E ~ 1-2 vs 10-12.5). This la rge discrepancy co uld be ex plained by recrystalli zation o f the ice during or shortl y a fter co ring. Estim ates of the ex pected ree r ystallization tim e-scale bracket th e ~I h time-sca le o f coring and leave the likelihood o f rec rys ta lli zation uncertain . H owever, the obse rved two-maximum fa bric type is not what is ex pected for a nnealing recrystalli zation fr om th e sha rp single-m ax imum fabric that wo uld be expec ted in situ at th e high shea r strains involved (r ~ 20). E xp erimental da ta from Wil so n (1982) suggest that, if the core did rec r ys ta lli ze, the pri or fa bric was a two-m ax imum fabric no t substa nti a ll y different from th e o bse rved one, whi ch impli es that th e m easured now law a nd deri ved m a rg in al shear stress a lT a pplicabl e to th e in situ situ ati o n. Th ese issues need to be resolved b y furth er work to o bta in a more definitive observa tio na l assess ment of the m a rgin al shear stress.

[1] New laboratory experiments exploring likely subglacial conditions reveal controls on the transition between stable sliding and stick-slip motion of debris-laden ice over rock, with implications for glacier behavior. Friction between a rock substrate and clasts in ice generates heat, which melts nearby ice to produce lubricating water. An increase in sliding speed or an increase in entrained debris raises heat generation and thus meltwater production. Unstable sliding is favored by low initial lubrication followed by rapid meltwater production in response to a velocity increase. Low initial lubrication can result from cold or drained conditions, whereas rapid increase in meltwater generation results from strong frictional heating caused by high sliding velocity or high debris loads. Strengthening of the interface (healing) during "stick" intervals between slip events occurs primarily through meltwater refreezing. When healing and unstable sliding are taken together, the experiments reported here suggest that stick-slip behavior is common from motion of debris-laden glacier ice over bedrock.