Atmospheric and basalt weathering cation inputs to soils and the development of secondary calcite and dolomite on the island of Hawaii

This study documents the development of Quaternary pedogenic, carbonate on the island of Hawai'i. The carbonate occurs as coatings and cement on 5000 to 350,000 year old basalt flows on northwestern Hawai'i, and as rhizoliths (root traces) and laminated sheets in <30,000 year old tephra deposits on a coastal site at South Point, the southernmost extension of Hawai'i. Both areas have been isolated from direct seawater and groundwater influence throughout their history. Although pedogenic carbonate usually contains <10% MgCO3, rhizoliths and sheets at South Point are composed of magnesian calcite (up to 14% MgCO3)Strontium isotopes and trace elements indicate that sea spray is an important cation source, providing up to ∼80% of labile Sr and ∼50% of carbonate Sr. The high Mg/Ca of sea spray (~5:1) led to soil waters with elevated Mg/Ca ratios and the precipitation of magnesian calcite. Nutrient elements, such as Ca, Mg, and K are traditionally thought to be derived primarily from rock weathering (e.g. Schlesinger, 1997). However, sea spray was found to be a major source of nutrient elements at South Point, supplying up to 2/3 of labile (soil water and exchangeable) Ca and 1/3 of carbonate Ca, and significant quantities of Mg and K as well. Magnesian calcite (up to 12% MgCO3) and stoichiometric dolomite were detected in soils on the northwestern basaltic flows. Magnesium content of carbonate increases with substrate age at these sites, suggesting alteration of early-formed low-Mg calcite to high-Mg calcite and finally to dolomite. Geological constraints and isotopic data confirm that these carbonate minerals are neither marine-influenced nor the result of alteration of a carbonate mineral-bearing parent material or eolian input. In contrast to continental calcretes, which often receive substantial input from eolian dust, strontium isotopes indicate >80% of northwestern Hawaiian carbonate Sr and Ca is derived from basalt. Weathering of this basalt, which has Mg/Ca ratios of 0.7--1.5, produced soil waters with similar Mg/Ca ratios, leading to precipitation of magnesian carbonate. Percolation of Mg-rich waters through early-formed calcite, perhaps aided by the boring and reworking by roots, led to formation of increasingly magnesian calcite and dolomite.