Weathering regime associated with subsurface circulation on volcanic islands

Abstract

International audienceVolcanic islands, being characterized by highly porous basaltic/andesitic lava flows and pyroclastic deposits, are subject to important chemical weathering by subsurface waters. Moreover, such subsurface weathering is impacted by hydrothermal springs in both active and non-active volcanic areas, thus increasing dissolved load concentrations. Here, we focus on the subsurface water chemistry in the volcanic islands of the Lesser Antilles and Re'union and on the origin of these subsurface flows. We are able, through the use of various isotopic tools (C, Sr, U-Th), to identify hydrothermal influences in river water. For example, Li concentrations show a positive correlation with temperature of hot and cold springs and also a relationship with d13C; from this, we can show that several sources of hydrothermal activity influence the rivers of the Lesser Antilles and that some rivers also reveal an important organic influence. As much as 20% of the subsurface hydrothermal springs go to feed the rivers. The increasing temperatures result in more dissolved elements being mobilized and an increase in chemical weathering rates. In addition, using the (230Th/238U) isochron for the well and river dissolved loads in Martinique, Guadeloupe and Re'union, we can evaluate residence times in the river water, i.e. the average residence time in the water along the circulation path to the sampling point. Alteration takes longer when the water circulates through thick soil, for example, 400-5,000 years when circulating under an ash profile and 1,200-15,000 years when circulating through a collapse zone. It would appear that waters circulation is globally three times longer for subsurface water than for surficial water. The weathering regime in tropical volcanic environments seems to be controlled mainly by such subsurface circulation with high chemical concentration from hydrothermal inputs. The origin of these compositions is varied and not controlled by a single hydrothermal spring. Consequently, it is subsurface circulation that determines the weathering regime in tropical volcanic islands with the main controlling parameters being temperature and residence time

Chromium fluxes and speciation in ultramafic catchments and global rivers

“…Some of the highest runoff values in this compilation are from volcanic regions (many located in tropical regions). The high runoff, along with the input of volcanic acids from hydrothermal sources (Rad et al, 2011) may explain why volcanic areas have the highest area normalized weathering rates recorded (Gaillardet et al, 2011).…”

Controls on the Hydrological and Topographic Evolution of Shield Volcanoes and Volcanic Ocean Islands

“…At the same time, subsurface chemical erosion progresses quickly as groundwater passes through the highly permeable and chemically reactive bedrock. High-temperature water-rock interactions and long subsurface residence times can further enhance chemical erosion rates [Rad et al, 2011]. Rad et al [2007] estimated subsurface chemical erosion rates from measurements of groundwater solute concentrations and estimates of groundwater recharge on the volcanic islands of Guadeloupe, Martinique, and Réunion.…”

Groundwater composition in perched-water bodies in the north flank of Fogo volcano (São Miguel, Azores): main causes and comparison with river water chemistry

“…Discharge is, in general, higher in springs discharging from lava flow aquifers, when compared with aquifers composed of pyroclastic deposits, and a study made in São Miguel island has shown that the average summer discharge in lava flow aquifers is about 5.6 9 10 -3 L/s, higher than the average value observed in springs from pyroclastic formations (3.82 9 10 -4 L/s; Paradela 1980). Studies on springs located in volcanic grounds have been made in dormant or active volcanoes worldwide, to characterize mineral water discharges (Shevenell and Goff 1993;Valentino and Stanzione 2003;Tassi et al 2003;Evans et al 2006;Minissale et al 2007), to calculate weathering rates (Rademacher et al 2005; Van der Weidjen and Pacheco 2003), to compute the total chemical weathering fluxes from volcanic islands (Rad et al 2011;Schopka and Derry 2012), and to define conceptual models about the functioning of groundwater systems (Irawan et al 2009). In the Azores, a few specific contributions have been published, mainly about groundwater composition (Cruz and Amaral 2004;Cruz and França 2006;Cordeiro et al 2012), chemical weathering rates (Freire et al 2014), diatom assemblages (Quintela et al 2013) and pelotherapy (Terroso 2005;Terroso et al 2006).…”