WHAT DO RARE EARTH ELEMENT DISTRIBUTIONS IN PELAGIC-AND HEMIPELAGIC SEQUENCES REVEAL ABOUT DEPOSITION AND DIAGENESIS?

Because of their distinctive chemical characteristics, the rare earth elements (REEs) behave systematically in marine environments and their associated distribution patterns “fingerprint” aspects of the ambient environment. The premise that marine chemical sediments may similarly archive environmental characteristics make REEs attractive as potential tracers, but there is little consensus about preservation. To investigate this system, we determined REE spectra from weak-acid soluble (labile) fractions on a continuous, 3 cm, basis throughout a 10 m core interval of the Monterey Formation. Within this setting, deposition oscillated between pelagic and hemipelagic modes in response to eustatic fluctuations, in turn related to global cooling episodes. Superimposed are significant diagenetic modifications including alteration of biogenic silica (diatoms), early authigenesis (dol, CFA, pyr) and organic matter (OM) decomposition. The sampled interval represents ~200 kyr of Late Miocene deposition and spans typical lithostratigraphic transitions (mudstone-porcelanite-dolomite) widely observed in the unit. Lack of bioturbation, high organic C contents, and negative δ¹³C dolomite compositions demonstrate that early diagenetic porewaters were anoxic near the sediment-water interface throughout the sampled sequence. Interestingly, shale-normalized REE patterns of labile fractions consistently resemble seawater features, notably heavy REE enrichment and marked Ce depletion (negative anomaly), regardless of host lithology. Such pronounced negative Ce anomalies are uncommon in reducing porewaters, thus here appear to persist due to buffering by biogenic phases (diatoms, microfossil calcite, OM) formed in the oxic water column. Ba (≤ 560 ppm) is notably abundant below and within dolomite intervals, and ICP-MS analysis requires correction for oxide/hydroxide overlaps to avoid false positive Eu anomalies. U concentrations (6.4 ppm avg; 32.5 ppm max) parallel the stratigraphic distribution of total REEs, and greatly exceed the range of marine carbonates, suggesting that early diagenetic porewaters contributed REEs to the labile fraction. Thus, despite the prominent role of diagenesis in this setting, REEs associated with labile phases appear to derive mainly from the oxic water column.