ON THE ORIGIN OF SALTPETER, NORTHERN CHILE COAST

Chile’s nitrate origin has been debated since Darwin visited Tarapaca deposits in 1835; deposits cover 6,500 Km2 of arid Atacama Desert, between Pisagua and Taltal.

N2 forms 70% of Earth’s atmosphere and is fixed by algae and bacteria before plants use; oceanic algae fix it in unknown amounts. NO3-(aq.) and NH4+(aq.) are main species in the N-O-H system Eh-pH stability diagram (25°C, 1 bar) and most common species in water; significant N2 water-solubility favors N accumulation in algae; NH4+ forms by algae decay; thus, nitrogen cycles in and out of algae biomass.

Nitrate deposits result from decay of marine algae concentrations living above the 9 Km-deep Peru-Chile trench, due to N, K, P plant nutrients cycling by sea currents and seawater uplift in the trench’s reduced gravity field; collapse waves from the +20m. high sea-rise, form abundant sea-spray, taken inland by SW winds; these facts link algae’s N2 biological uptake and accumulation with NH4+-rich sea-spray from algae decay, which when risen over the 2,000m-high Coast Range into the desert’s high solar radiation atmosphere undergoes photochemical oxidation of NH4+ to NO3-.

Nitrates accumulate on land both through marine-fog precipitation, and sea-spray oxidation/desiccation and gravitational settling of airborne NaNO3, KNO3, NaCl, Na2SO4, I, in the hot/dry desert atmosphere. El Niño/La Niña extreme aridity/torrential rain cycles favor nitrates accumulation through both aridity and water solution/remobilization/transportation onto slopes and into basins; capillary solution movement forms layers of nitrates; pure nitrate forms rare veins.

Nitrate deposits, as irregular strata >1m. thick, average 8% NaNO3 (range 3% - >13% NaNO3; mining cut-off grade is 7%). Nitrate ore reserves are estimated at 6,800x106 tons (equivalent to 544x106 tons NaNO3); accumulation rate would be 32.1 ton NaNO3/year (or 88 miligrams/m2/day) if it started in the Miocene, 17m.y. ago (extreme aridity onset); exploitation rate today is 106 tons NaNO3/year; 200x106 tons may have been exploited since 1809. Nitrates are non-renewable, since accumulation rate is very slow; efficient use of nitrate fertilizers could help reduce nitrates exhaustion risk.

Studies of natural nitrate formation processes and accumulation rate measurements would help define more efficient resource uses.