Calcium transfer and mass balance associated with soil carbonate in a semi‐arid silicate watershed (North Cameroon): An overlooked geochemical cascade?

Fabienne Dietrich,Pierre Deschamps,David Sebag,Nathalie Diaz,Eric P Verrecchia

doi:10.1002/dep2.134

Abstract

AbstractCalcium is a key element of the Earth system and closely coupled to the carbon cycle. Weathering of silicate releases Ca, which is exported and sequestered in oceans. However, pedogenic calcium carbonate constitutes a second Ca‐trapping pathway that has received less attention. Large accumulations of pedogenic calcium carbonate nodules, associated with palaeo‐Vertisols, are widespread in North Cameroon, despite a carbonate‐free watershed. A previous study suggested that a significant proportion of Ca released during weathering was trapped in palaeo‐Vertisols but the pathways involved in the transfer of Ca from sources (the granite and the Saharan dust) to a temporary sink (the carbonate nodules) remain unclear. This study aims to compare the distribution of elements in carbonate nodules and their associated past and present compartments for Ca in the landscape. These compartments are all characterised by a distinctive geochemical composition, resulting from specific processes. Three end members have been defined based on geochemical data: (a) the granite and its residual products, dominated by K2O and Na2O, Ti and Zr, HREE, and a positive Ce anomaly; (b) the soil parental material and the Saharan dust, dominated by Al2O3, Fe2O3 and MgO, V, HREE, and a positive Ce anomaly; and finally (c) the carbonate nodules, which are dominated by CaO, a depletion in V, Ti and Zr, and an enrichment in REE with a negative Ce anomaly. Mass balance calculations in soil profiles demonstrated that the accumulation of Ca in carbonate nodules exceeds the Ca released by chemical weathering of the parental material, because of a continuous accumulation and contribution from lateral transfers. Consequently, at the landscape scale, carbonate nodules associated with palaeo‐Vertisols constitute a temporary sink for Ca. Such a spatial relationship between sources and transient compartments opens an avenue to the new concept of ‘geochemical cascade’, similar in terms of geochemistry, to the concept of ‘sediment cascade’ developed by continental sedimentologists.

Highlights

Calcium is the fifth most abundant element in the Earth's crust (Rudnick & Gao, 2003) and is coupled to the carbon cycle at two different time scales (Tipper et al, 2016): (a) in the short-term carbon cycle, Ca is an important micronutrient for all organisms, for example, to maintain cell wall stability (Da Silva & Williams, 2001), and (b) in the long-term carbon cycle, Ca released during silicate weathering and sequestered as marine carbonate regulates atmospheric CO2 concentrations (Berner et al, 1983; Walker et al, 1981)
An alternative pathway may exist for Ca: it can be sequestered as pedogenic carbonate on continents, at least temporarily, and a significant part of the Ca released from silicate weathering is not directly transferred into the rivers towards oceans
The first group refers to the coarse alluvium (CAL, n = 13, samples from R and Q profiles), whereas the fersiallitic pedolith (FP) (T1–T6) constitutes the second group with a sample of recent alluvium (A1; n = 6)

Summary

Introduction

Calcium is the fifth most abundant element in the Earth's crust (Rudnick & Gao, 2003) and is coupled to the carbon cycle at two different time scales (Tipper et al, 2016): (a) in the short-term carbon cycle, Ca is an important micronutrient for all organisms, for example, to maintain cell wall stability (Da Silva & Williams, 2001), and (b) in the long-term carbon cycle, Ca released during silicate weathering and sequestered as marine carbonate regulates atmospheric CO2 concentrations (Berner et al, 1983; Walker et al, 1981). Defined as secondary calcium carbonate precipitations inside soils (Lal et al, 2000; Verrecchia, 2011), pedogenic carbonates may act as a long-term sink regarding C and Ca (Monger et al, 2015) and are an important part of the soil inorganic C (Eswaran et al, 2000; Zamanian et al, 2016). Processes leading to the formation of secondary calcium carbonate, for example, nodules, are still unclear (Zamanian et al, 2016), as well as their impact and role in the C and Ca coupled biogeochemical pathways and cycles

Objectives

Methods

Results

Conclusion