THE LATE DEVONIAN MARINE P CYCLE AND THE ORIGIN OF WIDESPREAD BLACK SHALES: ENHANCED RATES OF UPWELLING OR CHEMICAL WEATHERING?

Widespread marine black shales of Late Devonian age represent an enormous sink of organic C and P. The respective transient mass anomalies are estimated to be ca. 1-4 x 10¹⁹ mol and 2-10 x 10¹⁶ mol based on sediment mass, carbonate del¹³C, and C:P data. Does the scale of these anomalies place any constraints on processes of organic C and P accumulation? The marine P cycle has a relatively limited number of source and sink fluxes, principally river runoff and organic burial, respectively. Elevated P burial rates during the Late Devonian could reflect an increase in (1) the burial fraction, (2) the upwelling rate, or (3) the riverine flux of P. The first involves no change in marine productivity, whereas the latter two represent productivity stimuli with no change in the burial fraction. These mechanisms have different implications for evolution of the Late Devonian marine P cycle. Doubling of either the burial fraction or upwelling rate result in drawdown of seawater P to oligotrophic levels at timescales of ~10⁵ yr, making enhanced P burial rates difficult to sustain. Conversely, increases in the riverine P flux result in elevated seawater P and more eutrophic conditions that could have been sustained indefinitely through enhanced terrestrial weathering. Available evidence favors a model invoking increased marine productivity and weathering rate-climate feedbacks, with only a limited role for upwelling. First, increased Late Devonian productivity is documented by positive carbonate and organic C-isotopic shifts, C:P ratios, and frequent opportunistic marine algal blooms. Second, black shales are associated with eustatic highstands, whereas intensified upwelling is generally favored by cooler climates and eustatic lowstands. Third, onset of continental glaciation in the Late Famennian should have provided a positive feedback in an upwelling model, sustaining black shale deposition; instead, black shale deposition terminated abruptly near the Devono-Carboniferous boundary, a relationship more readily accounted for via a negative feedback, such as reduced rates of chemical weathering in response to global cooling. These considerations link elevated P burial fluxes to enhanced chemical weathering rates during the Late Devonian and may underscore the importance of rapid terrestrialization (i.e., spread of vascular land plants and soil formation) at that time.