TY - JOUR

T1 - Seasonal release of anoxic geothermal meltwater from the Katla volcanic system at Sólheimajökull, Iceland

AU - Wynn, Peter

AU - Morrell, David

AU - Tuffen, Hugh

AU - Barker, Philip

AU - Tweed, Fiona

AU - Burns, Rebecca

PY - 2015/3/9

Y1 - 2015/3/9

N2 - Understanding patterns of geothermal and volcanic activity at many of Iceland's most active volcanic systems is hampered by thick overlying ice, which prevents direct observation and complicates interpretation of geophysical signals. Katla is a prime example, being a large and restless volcanic system covered by the 740 m thick Mýrdalsjökull ice cap, whose eruptions have triggered some of the most powerful known meltwater floods in historical times. To shed new light on geothermal and subglacial hydrological processes at Katla, we have determined the sulphate isotopic composition of a series of glacial meltwater samples discharged from Sólheimajökull, a valley glacier of Mýrdalsjökull, between 2009 and 2012. Dual isotopic analysis of δ34S and δ18O in dissolved sulphate allows identification of source mixing processes and chemical evolution during subglacial meltwater transport. Strikingly, meltwater δ18OSO4 signatures indicate redox conditions at the glacier bed, which are inverse to those normally encountered at Arctic and Alpine glaciers. Discharge of reduced, anoxic meltwater in summer, rather than winter, points towards seasonal release of geothermally derived volatile gases. We attribute this to headward expansion of the channelized subglacial drainage system during the summer melt season, accessing key areas of geothermal activity within the Katla caldera. Volatile release may be further enhanced by unloading of overburden pressure due to snowpack melting in the summer season. In winter, restriction of subglacial channels to lower elevations effectively seals geothermal fluids and dissolved gases beneath the ice cap, with only sporadic release permitted by periodic increases in subglacial water pressure. When the subglacial drainage configuration permits access to key geothermal areas, sulphate isotopic signatures thereby form sensitive indicators of geothermal activity occurring deep beneath the Mýrdalsjökull ice cap.

AB - Understanding patterns of geothermal and volcanic activity at many of Iceland's most active volcanic systems is hampered by thick overlying ice, which prevents direct observation and complicates interpretation of geophysical signals. Katla is a prime example, being a large and restless volcanic system covered by the 740 m thick Mýrdalsjökull ice cap, whose eruptions have triggered some of the most powerful known meltwater floods in historical times. To shed new light on geothermal and subglacial hydrological processes at Katla, we have determined the sulphate isotopic composition of a series of glacial meltwater samples discharged from Sólheimajökull, a valley glacier of Mýrdalsjökull, between 2009 and 2012. Dual isotopic analysis of δ34S and δ18O in dissolved sulphate allows identification of source mixing processes and chemical evolution during subglacial meltwater transport. Strikingly, meltwater δ18OSO4 signatures indicate redox conditions at the glacier bed, which are inverse to those normally encountered at Arctic and Alpine glaciers. Discharge of reduced, anoxic meltwater in summer, rather than winter, points towards seasonal release of geothermally derived volatile gases. We attribute this to headward expansion of the channelized subglacial drainage system during the summer melt season, accessing key areas of geothermal activity within the Katla caldera. Volatile release may be further enhanced by unloading of overburden pressure due to snowpack melting in the summer season. In winter, restriction of subglacial channels to lower elevations effectively seals geothermal fluids and dissolved gases beneath the ice cap, with only sporadic release permitted by periodic increases in subglacial water pressure. When the subglacial drainage configuration permits access to key geothermal areas, sulphate isotopic signatures thereby form sensitive indicators of geothermal activity occurring deep beneath the Mýrdalsjökull ice cap.

KW - Iceland

KW - Mýrdalsjökull

KW - Sólheimajökull

KW - Geothermal water

KW - Glacial hydrochemistry

KW - Sulphate isotopes

U2 - 10.1016/j.chemgeo.2014.12.026

DO - 10.1016/j.chemgeo.2014.12.026

M3 - Journal article

VL - 396

SP - 228

EP - 238

JO - Chemical Geology

JF - Chemical Geology

SN - 0009-2541

ER -