Biology
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.v13h..05s&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #V13H-05
Biology
[1015] Geochemistry / Composition Of The Core, [1060] Geochemistry / Planetary Geochemistry, [3630] Mineralogy And Petrology / Experimental Mineralogy And Petrology, [5215] Planetary Sciences: Astrobiology / Origin Of Life
Scientific paper
The Earth abundance of the heavier halogens, Cl, Br and I, are significantly depleted relative to expected values based on CI chondrites and solar abundances. The cause of these ‘selective’ depletions may be related to 1) far greater volatility than previously assumed; 2) a hidden reservoir on Earth; 3) selective loss of the halogens during planetary accretion. The volatility of an element is related to its temperature of condensation from the cooling solar nebula. The high condensation temperature of Cl is based on sodalite crystallization at ~800 K (10-4 bar), but even if sodalite formation is kinetically impeded at such low pressures, NaCl (g) should condense to NaCl (s) at only slightly lower temperatures. An unreasonably low condensation temperature of ~200 K would be needed to explain Earth’s concentration of halogens. A second possibility for the apparent Earth depletion is that Cl is strongly partitioned into the core. We tested this hypothesis by experimentally measuring Cl partitioning between basalt and Fe (and Fe95.5S4.5) at high pressures and temperatures. Samples were doped with trace FeCl2 as a Cl source. The measured D (Cl) metal-basalt is less than 0.01 under all conditions, ruling out the possibility of a Cl sink in the core. We propose instead that the halogens were lost during the late giant bombardment stage of planetary accretion. The selective loss of the halogens relative to other elements with similar condensation temperatures is explained by their unique hydrophilic character. Early in Earth’s history, halogens were strongly partitioned into the ocean/surficial environment. They were then removed by atmospheric erosion associated with giant impacts towards the end of planetary accretion. Our results provide independent evidence for multiple atmospheric-loss events, a controversial conclusion that is at odds with some geophysical studies. Over 90% of Cl was lost in early Earth history. Today, the oceans host nearly half of Earth’s halogens. Had this massive removal of halogens not occurred, Earth’s oceans would be at halite saturation, roughly equivalent to the Dead Sea. The Earth would have suffered from ‘halogen poisoning’, and life may never have emerged under such conditions. Even if it had, evolution to more complex forms - associated with a drop in salinity and increase in dissolved oxygen (Knauth, Nature, 1998) - would almost certainly never have occurred. Evaporation over a halite-saturated ocean would be one half the modern value, so that precipitation would be drastically reduced or non-existent and terrestrial life would have been severely restricted. Mars, with its higher Cl and lower water contents (Filiberto and Trieman, 2009), would produce oceans with far higher salinities. Atmospheric erosion in chondritic-like star systems may be a necessary condition for life by preventing the formation of halogen-poisoned planets.
Draper David S.
Sharp Zachary D.
No associations
LandOfFree
The Chlorine Abundance of Earth: Evidence for Early Atmospheric Loss and Creation of a Life-Supporting Planet does not yet have a rating. At this time, there are no reviews or comments for this scientific paper.
If you have personal experience with The Chlorine Abundance of Earth: Evidence for Early Atmospheric Loss and Creation of a Life-Supporting Planet, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and The Chlorine Abundance of Earth: Evidence for Early Atmospheric Loss and Creation of a Life-Supporting Planet will most certainly appreciate the feedback.
Profile ID: LFWR-SCP-O-1776230