Shimmield, G.B.

English

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

The geochemistry and mineralogy of Pacific sediments, Baja California.

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