XVI INQUA Congress
Paper No. 6-9
Presentation Time: 1:30 PM-4:30 PM


ARIAS, Jaime, The Oregon and Chilean Exploration & Mining Co, El Alba 1, Parcela 18, Chicureo, Colina, Santiago Chile, m_oregon@entelchile.net.

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.

XVI INQUA Congress
General Information for this Meeting
Session No. 6--Booth# 116
Quaternary Economic Deposits and Landscape Evolution in the Late Cenozoic (Posters)
Reno Hilton Resort and Conference Center: Pavilion
1:30 PM-4:30 PM, Thursday, July 24, 2003

Geological Society of America Abstracts with Programs, , p. 82

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