Did the Monterey Formation contribute to Miocene climate change? - the question cannot be answered without considering location and facies

Abstract

Great inter- and intrabasinal variation in thickness and composition of the Miocene Monterey Formation obscures the possible role of the deposit in global geochemical balances and past climate change and therefore challenges previous conclusions that it was not significant. The “Monterey Hypothesis” of Vincent and Berger (1985) proposed that organic carbon accumulated and was preserved at such a high rate in the Miocene Monterey Formation and its stratigraphic equivalents that it perturbed the global carbon cycle and Earth shifted from a “greenhouse” to an “icehouse” world. This concept was later disputed by a number of studies that found that: a) the mass accumulation rates of organic carbon and other biochemically important elements (Si, P) were not appreciably higher than in modern upwelling systems and so could not have had an outsized global influence, and b) the isotopic records of cooling into a glacial world lagged the carbon isotopic evidence for organic matter burial by too long to be explained by a causal relationship. These arguments may not hold for several reasons: 1. All of the Monterey studies were conducted in stratigraphic sections on the central California coast where sediment accumulation rates were relatively slow due to their distal, outboard position in the Miocene continental borderland compared to more proximal basins. 2. Local, intrabasinal variation in Monterey depositional environment (banktop, slope or basin) produced up to 5x’s thickening of certain hemipelagic chronostratigraphic units in <10 miles, and the previously analyzed locations were all in thinner successions that contained significant condensed sections or unconformities. 3. Ancient sedimentation rates cannot be directly compared to modern ones because of the well-documented incompleteness of the stratigraphic record at all measurement scales. 4. The effects of abundant organic carbon burial may have been masked by increased global volcanism and CO2 emission, at least during deposition of the lower Monterey. These uncertainties and possibilities cry out for new chemostratigraphic work to be done at the basin-scale or in expanded outcrop and subsurface sections of the San Joaquin, Salinas or Los Angeles basins and even in thicker depocenters of the more distal Santa Barbara and Santa Maria basins in order to accurately evaluate the contribution of the Monterey Formation to global geochemical balances during the Miocene.

AAPG Datapages/Search and Discovery Article #90372 © 2020 AAPG Pacific Section Convention, 2020 Vision: Producing the Future, Mandalay Beach, Oxnard, CA, April 4-8, 2020 (Cancelled)