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The post-depositional cycling of Cd, Cu, Mo and Zn in several hydrographically distinct B.C. fjords

University of British Columbia

Sediment cores from six inlets representing markedly different hydrographic regimes along the British Columbia coast were collected to acquire high-resolution distributions of both solidphase and interstitial metals in addition to an extensive suite of ancillary data. A technique for small-volume trace metal extraction (<1 mL) was developed to remove the metals Cd, Cu, Mo and Zn from the salt-water matrix prior to analysis. The cycling of these metals was studied in detail within the context of the hydrographies of the environment presented by the various inlets. By observing and comparing the behaviour of each metal under different depositional conditions, the relative influence of hydrography on each element was characterized. The solid-phase inventories of the four elements broadly fall into one of two categories: those deplete in terrestrial detritus (Cd and Mo) and those enriched in lithogenic material (Cu and Zn). The distribution of the former two are influenced most markedly by post-depositional diagenesis while the latter two respond more to detrital inputs. Cadmium is introduced to many deposits in association with biogenic material; its release and efflux to bottom waters is attenuated by lower redox potential. Where sulphate reduction nears the sediment-water interface, the efflux is minimized as liberated Cd is converted to authigenic sulphides. Consequently, enhanced accumulation occurs under conditions of high primary productivity, low sedimentation rate and restricted estuarine circulation. Molybdenum is introduced most commonly in association with Mn-oxides, or, under sub-oxic regimes, by diffusion across the sediment-water interface followed by precipitation as an authigenic sulphide. In oxic core-tops, there exists a strong association between Mo and Mn-oxides; however, high sedimentation rates or periods of dysaerobia inhibit disproportionation of Mn thereby decoupling the relationship. Whole-core accumulation of Mo occurs only under conditions of interfacial suboxia/ anoxic. Consequently, the oceanographic factors which most markedly influence the accumulation of Mo (like Cd) are sedimentation rate and redox potential. Elevated sedimentation rates enhance the dilution of these elements by increasing the accumulation of lithogenic material which is deplete in both metals. Lower redox potentials enhance the accumulation of authigenic sulphides. Copper and zinc behave similarly in many coastal deposits; they are both elevated in lithogenic detritus. Both metals are associated with post-depositional release from organic matter oxidation, and display strong associations with diagenetic oxides; a preferential association with Mn oxides shifts to Fe oxides under less aerobic conditions. Both Cu and Zn display strong associations with both solid and dissolved sulphide species; however, unlike Cd or Mo, anoxia does not necessarily translate to elevated sedimentary concentrations. Lithogenic sources appear to be more important. The hydrographic factor which most influences the inventory of these elements is sedimentation rate (the primary influence on terrestrial inputs), while redox potential (.fundamentally controlled by organic carbon content and bottom water oxygen) exerts the greatest post-depositional influence on cycling. Anthropogenic inputs affect the inventory of all four metals (Mo to a lesser degree), but appear to have little influence on the degree of post-depositional cycling. Paradoxically, anthropogenic loadings of nutrients, manifested as eutrophication, exerts a greater influence on the cycling of trace metals.

Thesis/Dissertation

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2009-03-31

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http://hdl.handle.net/2429/6667

Environmental Sciences

University of British Columbia

UBCV

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