Direct evidence of the feedback between climate and weathering

Abstract

Long-term climate moderation is commonly attributed to chemical weathering; the higher the temperature and precipitation the faster the weathering rate. Weathering releases divalent cations to the ocean via riverine transport where they promote the drawdown of CO₂ from the atmosphere by the precipitation and subsequent burial of carbonate minerals. To test this widely-held hypothesis, we performed a field study determining the weathering rates of 8 nearly pristine north-eastern Iceland river catchments with varying glacial cover over 44 years. The mean annual temperature and annual precipitation of these catchments varied by 3.2 to 4.5 °C and 80 to 530%, respectively during the study period. Statistically significant linear positive correlations were found between mean annual temperature and chemical weathering in all 8 catchments and between mean annual temperature and both mechanical weathering and runoff in 7 of the 8 catchments. For each degree of temperature increase, the runoff, mechanical weathering flux, and chemical weathering fluxes in these catchments are found to increase from 6 to 16%, 8 to 30%, and 4 to 14% respectively, depending on the catchment. In contrast, annual precipitation is less related to the measured fluxes; statistically significant correlations between annual precipitation and runoff, mechanical weathering, and chemical weathering were found for 3 of the least glaciated catchments. Mechanical and chemical weathering increased with time in all catchments over the 44 year period. These correlations were statistically significant for only 2 of the 8 catchments due to scatter in corresponding annual runoff and average annual temperature versus time plots. Taken together, these results 1) demonstrate a significant feedback between climate and Earth surface weathering, and 2) suggest that weathering rates are currently increasing with time due to global warming.

Introduction

Atmospheric circulation models indicate that global climate is intimately linked to the atmospheric CO₂ content; increased CO₂ leads to warmer temperatures, changing precipitation patterns, and an overall increase in runoff (Labat et al., 2004, Gedney et al., 2006, Alley, 2007). Increased CO₂ content in the atmosphere, however, has been limited over geologic time by weathering of silicate rocks (Walker et al., 1981, Berner et al., 1983, Berner and Kothavala, 2001, Wallmann, 2001, Berner, 2004). This process stems from the weathering of Calcium–Magnesium silicates, and over the last 100 million years mainly Ca silicates. For example, the weathering of Ca-rich plagioclase, a common silicate mineral, leads to carbonate mineral precipitation according to the following reactions:

CaAl₂Si₂O₈ + 2CO₂ + 3H₂O→

Plagioclase

Al₂Si₂O₅(OH)₄ + Ca²⁺ + 2HCO₃⁻

Kaolinite

Ca²⁺ + 2HCO₃⁻ → CaCO₃ + CO₂ + H₂O

Calcite

These reactions proceed via the chemical and mechanical weathering of silicates on land coupled to the riverine transport of suspended matter and aqueous Ca²⁺ and HCO₃⁻ to the oceans where they react to form calcite and aragonite (Aller, 1998, Gislason and Oelkers, 2003, Stefánsdóttir and Gislason, 2005, Gislason et al., 2006, Pogge von Strandmann et al., 2008). The feedback between climate and weathering has been attributed to a number of factors including the effect of temperature on 1) silicate dissolution rates, 2) runoff, 3) mechanical weathering, and 4) glacial melting (e.g. Peters, 1984, Meybeck, 1986, Sverdrup, 1990, Bluth and Kump, 1994, Meybeck, 1994, Gibbs and Kump, 1994, White and Blum, 1995, Gislason et al., 1996, Gaillardet et al., 1999a, Stefánsson and Gislason, 2001, Tranter et al., 2002, Dessert et al., 2003, Millot et al., 2003, West et al., 2005, Anderson, 2007, Navarra-Sitchler and Brantley, 2007).

This study was initiated to provide direct evidence of the effect of climate change on chemical and mechanical weathering rates. This evidence was obtained by focusing on individual catchments rather than comparing the behaviour of different catchments at various temperatures and precipitation/runoffs. In this way we were able to measure directly changes in weathering rates due to climate change in each catchment, limiting potential ambiguities associated with relief, rock type, vegetation, and glacier cover, etc., which could mask the effects of temperature and precipitation/runoff on weathering (Edmond et al., 1995).

Chemical and mechanical weathering was quantified by measuring the flux of dissolved and suspended materials, respectively, to the ocean in 8 rivers in north-eastern Iceland. The location of these rivers and their catchments are shown in Fig. 1; further physical and climatic characteristics of these catchments are presented in Table 1. The 8 studied rivers are situated on basaltic rocks. The ages of these basalts generally increase from east to west, the youngest basalts are found in the Jökulsá á Fjöllum river catchment, and are on average 0.3 million years of age. The oldest basalts are found in the Fjardará river catchment and are on average 11.2 million years of age (Moorbath et al., 1968, Johannesson and Saemundsson, 1998). Five of the eight river catchments are partially glaciated. Two of the rivers, the Lagarfljót and Grímsá, have been dammed. Critical to demonstrating the effect of climate on weathering is performing studies over a sufficiently long time period to obtain data exhibiting distinct mean annual temperature and precipitation changes. To obtain the robust evidence needed to demonstrate unambiguously the feedback between climate and weathering, this study covers the years 1961–2004 corresponding to most of the recent rapid global warming period (Alley et al., 2007).

The north-east Icelandic rivers were chosen for this study for a number of reasons. First, these river catchments are sparsely populated, minimizing the effect of human activity on weathering rates. Second, the river catchments are comprised of basalts. Due to their composition and rapid dissolution rates, the weathering of continental basalts has a far faster weathering rate and CO₂ consumption capacity than other major continental silicate rocks (Meybeck, 1986, Bluth and Kump, 1994, Dessert et al., 2003, Wolff-Boenisch et al., 2006). Third, Iceland is a volcanic island representative of the high-relief, volcanic and tectonically active islands that contribute over 45% of suspended material to the oceans, and the suspended material from these islands is reactive in seawater (Milliman and Syvitski, 1992, Gaillardet et al., 1999b, Stefánsdóttir and Gislason, 2005, Gislason et al., 2006). Fourth, future warming and rainfall increase is expected to be greatest over land situated at high latitudes (Alley et al., 2007). Fifth, the studied rivers consist of 5 glacier-fed and 3 non-glacial rivers, allowing assessment of the role of glacial melting on weathering rates. Finally, modelling studies have suggested that the effect of climate on weathering would be greatest in volcanic rocks (Wallmann, 2001) making this among the best sites to observe this effect.