Geochemistry and mineralogy of Pacific sediments, Baja California

Abstract

The geochemical composition, mineralogy and radiochemistry of a suite of box-cores traversing the Pacific margin of the Baja California peninsula has been investigated. In particular, the role of diagenesis in influencing geochemical transformations is assessed.

The Baja California margin is situated at the confluence of the California Current and California Undercurrent. Resulting upwelling and high surface productivity results in a substantial flux of organic matter to the shelf and hemipelagic environment.

Detrital element geochemistry (Si, Zr, K, Mg, Fe, Rb) and mineralogy indicates that clay contents increase oceanwards, the hemipelagic and pelagic environment being dominated by smectite. Within hemipelagic sediments, grain size increases with depth (>20 cm), attributed to enhanced detrital sedimentation during the last glacial episode. Biogenic elements (Ca, Sr) are also elevated in glacial sediment although the enrichment may be due to either increased biological productivity or an increase in detrital carbonate. Ba is believed to result from biological vectoring but is diluted in concentration in glacial sediment.

Organic matter diagenesis results in a sequence of diagenetic zones within hemipelagic and pelagic sediment. With increasing diagenesis N, I and Br are more labile than in the fully oxic pelagic sediment where clay fixation is prominent. On the shelf, heavy metals (Cr, Ni, Cu, Zn, Mo, Pb), do not appear to be associated with the high levels of organic matter found, whilst grain size effectively controls the depth distribution of organic C.

The remobilisation of Mn and associated trace metals (Ni, Cu, Zn, Mo) during diagenesis is well displayed in hemipelagic sediments. The depth to the onset of Mn reduction occurs within bioturbated sediment defined by the distribution of ²³⁰Th and ²³¹Pa.

The oxidation state of the Mn is always high (~MnO1.8) above the redoxcline although reduction in microenvironments at the sediment/ water interface results in slightly lower oxidation states (~MnO₁.₇₅). The level of trace metal enrichment is Mo>Ni>Zn=Cu, the same as their relative concentrations in seawater. With increasing diagenesis Ni decreases relative to Mn⁴⁺. Following Mn reduction smectite absorbtion of released trace metals is important. Mo is lost prior to Mn remobilisation probably to the dissolved organic carbon pool or to monosulphide precipitation.

Measured ²³⁰Thₑₓₑₑₛₛ /²³¹Paₑₓₑₑₛₛ activity ratios decrease monotonically landwards. This is indication of preferential ²³¹Pa removal at ocean margins. With time the flux of particles has varied, resulting in changing initial activity ratios within the sediment. Long-lived isotopes are therefore unsuitable to the measurement of sedimentation rate in areas subject to variable particle flux.

Diagenetic mineral formation occurs in a variety of depositional environments. The precipitation of both apatite and manganese carbonate occurs in coarser-grained sediment, apatite forming on the shelf resulting from PO₄³⁻ generation during organic matter decomposition, and manganese carbonate forming at depth in the hemipelagic sediment after reduction of MnO₂. Glaucony forms at sites of local reduction prior to SO₄²⁻ reduction. Dolomite forms in slowly-accumulating, organic-rich sediments of the hemipelagic and slope environments prior to extensive SO₄²⁻ reduction. Pyrite formation is an important sink for Fe in reducing shelf sediments.