Phosphogenesis and organic-carbon preservation in the Miocene Monterey Formation at Naples Beach, California—The Monterey hypothesis revisited

Abstract

The middle part of the Miocene Monterey Formation at Naples Beach, west of Santa Barbara, California, is predominantly composed of organic-rich mudstone interstratifi ed with phosphatic laminae. Minor lithologies include volcanic ash, dolomite, porcelanite and chert, and condensed phosphatic beds. Sediments dated as 14.3-13.5 Ma have average total organic carbon (TOC) values around 8.5 wt%, and organic carbon (OC) accumulation rates are around 565 mg/cm 2 /k.y. Sediments dated as 13.5-13 Ma are characterized by average TOC values of 12.6 wt% and OC accumulation rates of around 1130 mg/cm 2 / k.y. The interval between 13 and 10.6 Ma is marked by condensation; average TOC values are around 8.6 wt%, and OC accumulation rates diminished to around 55 mg/cm 2 /k.y. The last interval studied is dated as 10.6-9.4 Ma, and average TOC values are around 6 wt%, whereas OC accumulation rates rose again to 320 mg/cm 2 /k.y.The presence of erosional surfaces, angular unconformities, and reworked clasts and nodules suggests that bottom-current activity and gravity-fl ow deposition have been instrumental in sediment accumulation. The phosphatic laminae were precipitated at a very early stage of diagenesis during periods of nonsedimentation. They formed less permeable sedimentary lids and may as such have contributed to enhanced OC preservation. Between 13 and 10.6 Ma, the thusformed phosphatic laminae were frequently subjected to subsequent sediment winnowing and reworking, resulting in the formation of condensed phosphatic beds. Calculated P:C molar ratios suggest that (1) the measured section is highly enriched in phosphorus (P) relative to OC; (2) regeneration of organic P from organic-matter decomposition was negligible; and (3) the source of P was external, likely upwelled bottom water rich in inorganic P.In spite of good preservation conditions and correspondingly high TOC contents, the overall OC accumulation rates are moderate in comparison to those of actual high productivity areas, which is mainly due to the episodic character of depositional processes and the intervening long periods of nondeposition and sediment reworking. They preclude this section, and by extrapolation, the Monterey Formation in general from being an important OC sink during the middle Miocene. Alternatively, large OC sinks were probably created on the continent (lignite deposits) and in sedimentary depocenters, which received increasing amounts of detrital sediments due to a combination of climate change, spreading of grasslands, and the increasing importance of mountain chains such as the Himalaya. The associated high nutrient fl uxes may have been involved in the backstepping and drowning of carbonate platforms and in the generation of widespread phosphate-rich deposits during the late early and early middle Miocene.

Organic carbon deposition and phosphorus accumulation during Oceanic Anoxic Event 2 in Tarfaya, Morocco

“…P detrital is susceptible to having been diagenetically transformed from P authigenic (e.g. Filippelli and Delaney, 1995;Fö llmi et al, 2005). It also in no way correlates with the detrital index described above.…”

Determining the Diagenetic Conditions of Concretion Formation: Assessing Temperatures and Pore Waters Using Clumped Isotopes

“…The Monterey Formation contains evidence of a diverse diagenetic history-involving phosphate-mineral genesis (e.g., Garrison et al 1987;Fö llmi et al 2005), silica polymorphism (e.g., Behl and Garrison 1994), organic-matter degradation and/or preservation (e.g., John et al 2002;Loyd et al 2012), and concretion precipitation (e.g., Bramlette 1946;Kushnir and Kastner 1984;Hennessy and Knauth 1985)-aspects of which probably were driven by or led to its high organic content (up to 25% by weight; Isaacs 2001). Because of this high organic content, the Monterey Formation is an important source and reservoir rock for petroleum in California (Surdam and Stanley 1981;Mertz et al 1983;Isaacs 1981).…”

Phosphorus burial and diagenesis in the central Bering Sea (Bowers Ridge, IODP Site U1341): Perspectives on the marine P cycle

“…In conclusion, a significant portion of CFA at Site U1341 appears to be terrigenous despite the open marine location of Bowers Ridge, and this seems to be true for marine deposits from other climatic and depositional settings as well (Föllmi, 1996 and references therein; Compton et al, 2000;Föllmi et al, 2005;Kraal et al, 2012).…”