Editing Geochemistry (section)

==Trace metals in the ocean==
[[Trace metal]]s readily form [[Complex (chemistry)|complexes]] with major [[ion]]s in the ocean, including [[hydroxide]], [[carbonate]], and [[chloride]] and their chemical speciation changes depending on whether the environment is [[redox|oxidized or reduced]].<ref>{{cite journal|last1=Nameroff|first1=T|last2=Balistrieri|first2=L|last3=Murray|first3=J|title=Suboxic Trace Metal Geochemistry in the Eastern Tropic North Pacific|journal=Geochimica et Cosmochimica Acta|volume=66|issue=7|pages=1139–1158|date=2002|doi=10.1016/s0016-7037(01)00843-2|bibcode=2002GeCoA..66.1139N}}</ref>  Benjamin (2002) defines complexes of metals with more than one type of [[ligand]], other than water, as mixed-ligand-complexes. In some cases, a ligand contains more than one ''donor'' atom, forming very strong complexes, also called [[chelate]]s (the ligand is the chelator). One of the most common chelators is EDTA ([[ethylenediaminetetraacetic acid]]), which can replace six molecules of water and form strong bonds with metals that have a plus two charge.<ref name=Benjamin>{{cite book|last1=Benjamin|first1=M|title=Water Chemistry|date=2002|publisher=University of Washington|isbn=1-57766-667-4}}</ref>  With stronger complexation, lower [[Thermodynamic activity|activity]] of the free metal ion is observed. One consequence of the lower reactivity of complexed metals compared to the same concentration of free metal is that the chelation tends to stabilize metals in the aqueous solution instead of in solids.<ref name=Benjamin/>
 
[[Chemical concentration|Concentrations]] of the trace metals [[cadmium]], [[copper]], [[molybdenum]], [[manganese]], [[rhenium]], [[uranium]] and [[vanadium]] in sediments record the redox history of the oceans.<ref name=":0">{{Cite journal |last1=Ferriday |first1=Tim |last2=Montenari |first2=Michael |date=2016 |title=Chemostratigraphy and Chemofacies of Source Rock Analogues: A High-Resolution Analysis of Black Shale Successions from the Lower Silurian Formigoso Formation (Cantabrian Mountains, NW Spain) |url=https://www.sciencedirect.com/science/article/abs/pii/S2468517816300053 |journal=Stratigraphy & Timescales |volume=1 |pages=123–255 |doi=10.1016/bs.sats.2020.07.001 |s2cid=229217907 |via=Elsevier Science Direct |access-date=2022-06-20 |archive-date=2022-06-20 |archive-url=https://web.archive.org/web/20220620140355/https://www.sciencedirect.com/science/article/abs/pii/S2468517816300053 |url-status=live }}</ref> Within aquatic environments, cadmium(II) can either be in the form CdCl<sup>+</sup><sub>(aq)</sub> in [[oxic]] waters or CdS(s) in a reduced environment. Thus, higher concentrations of Cd in marine sediments may indicate low redox potential conditions in the past. For copper(II), a prevalent form is CuCl<sup>+</sup>(aq) within oxic environments and CuS(s) and Cu<sub>2</sub>S within reduced environments. The reduced seawater environment leads to two possible oxidation states of copper, Cu(I) and Cu(II).<ref name=":0" /> [[Molybdenum]] is present as the Mo(VI) oxidation state as MoO<sub>4</sub><sup>2−</sup><sub>(aq)</sub> in oxic environments. Mo(V) and Mo(IV) are present in reduced environments in the forms MoO<sub>2</sub><sup>+</sup><sub>(aq)</sub> and MoS<sub>2(s)</sub>.<ref name=":0" /> [[Rhenium]] is present as the Re(VII) oxidation state as ReO<sub>4</sub><sup>−</sup> within oxic conditions, but is reduced to Re(IV) which may form ReO<sub>2</sub> or ReS<sub>2</sub>. Uranium is in oxidation state VI in UO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub><sup>4−</sup>(aq) and is found in the reduced form UO<sub>2</sub>(s).<ref name=":0" /> [[Vanadium]] is in several forms in oxidation state V(V); HVO<sub>4</sub><sup>2−</sup> and H<sub>2</sub>VO<sub>4</sub><sup>−</sup>. Its reduced forms can include VO<sub>2</sub><sup>+</sup>, VO(OH)<sub>3</sub><sup>−</sup>, and V(OH)<sub>3</sub>.<ref name=":0" /> These relative dominance of these species depends on [[pH]].

In the water column of the ocean or deep lakes, vertical profiles of dissolved trace metals are characterized as following ''conservative–type'', ''nutrient–type'', or ''scavenged–type'' distributions. Across these three distributions, trace metals have different residence times and are used to varying extents by [[plankton]]ic microorganisms. Trace metals with conservative-type distributions have high concentrations relative to their biological use. One example of a trace metal with a conservative-type distribution is molybdenum. It has a residence time within the oceans of around 8 x 10<sup>5</sup> years and is generally present as the [[molybdate]] anion (MoO<sub>4</sub><sup>2−</sup>). Molybdenum interacts weakly with particles and displays an almost uniform vertical profile in the ocean. Relative to the abundance of molybdenum in the ocean, the amount required as a metal cofactor for [[enzyme]]s in marine [[phytoplankton]] is negligible.<ref name=Bruland>{{cite book |last1=Bruland |first1=K |last2=Lohan |first2=M |chapter= 6.02 &ndash; Controls on Trace Metals in Seawater|editor-last1=Holland |editor-first1=H.D. |editor-last2= Turekian |editor-first2=K.K. |title= Treatise on Geochemistry |volume= 6: The Oceans and Marine Geochemistry |date=2003 |pages=23–47 |doi= 10.1016/B0-08-043751-6/06105-3|bibcode=2003TrGeo...6...23B}}</ref>

Trace metals with nutrient-type distributions are strongly associated with the internal cycles of particulate organic matter, especially the assimilation by plankton. The lowest dissolved concentrations of these metals are at the surface of the ocean, where they are assimilated by [[plankton]]. As dissolution and decomposition occur at greater depths, concentrations of these trace metals increase. Residence times of these metals, such as zinc, are several thousand to one hundred thousand years. Finally, an example of a scavenged-type trace metal is [[aluminium]], which has strong interactions with particles as well as a short residence time in the ocean. The residence times of scavenged-type trace metals are around 100 to 1000 years. The concentrations of these metals are highest around bottom sediments, [[hydrothermal vent]]s, and rivers. For aluminium, atmospheric dust provides the greatest source of external inputs into the ocean.<ref name=Bruland/>

Iron and copper show hybrid distributions in the ocean. They are influenced by recycling and intense scavenging. Iron is a limiting nutrient in vast areas of the oceans and is found in high abundance along with manganese near hydrothermal vents. Here, many iron precipitates are found, mostly in the forms of iron sulfides and oxidized iron oxyhydroxide compounds. Concentrations of iron near hydrothermal vents can be up to one million times the concentrations found in the open ocean.<ref name=Bruland/>

Using electrochemical techniques, it is possible to show that bioactive trace metals (zinc, cobalt, cadmium, iron, and copper) are bound by organic ligands in surface seawater. These ligand complexes serve to lower the bioavailability of trace metals within the ocean. For example, copper, which may be toxic to open ocean phytoplankton and bacteria, can form organic complexes. The formation of these complexes reduces the concentrations of bioavailable inorganic complexes of copper that could be toxic to sea life at high concentrations. Unlike copper, zinc toxicity in marine phytoplankton is low and there is no advantage to increasing the organic binding of Zn<sup>2+</sup>. In [[high-nutrient, low-chlorophyll regions]], iron is the limiting nutrient, with the dominant species being strong organic complexes of Fe(III).<ref name=Bruland/>