--> Correlation of Organic Carbon Deposition Within the Monterey Formation to The Miocene Monterey Excursion Event in the Eastern Pacific Margin

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Correlation of Organic Carbon Deposition Within the Monterey Formation to The Miocene Monterey Excursion Event in the Eastern Pacific Margin


The Monterey Formation is an extremely complex lithostratigraphic unit consisting of siliceous and calcareous biogenic sediments deposited in marginal basins formed during a tectonic reorganization of the California borderland in the late Oligocene to early Miocene. As the marginal basins subsided and sea level rose in late early Miocene, terrigenous-rich sedimentation was replaced by the deposition of calcareous and bio-siliceous marls that marked the start of Monterey deposition (Issacs 1983). The deposition of the Monterey Formation occurred during an important time in the climate and oceanic evolution of the mid to late Cenozoic; the transition from a relatively warm greenhouse climate in the early Miocene to cooler temperatures of the developing icehouse climatic conditions during the middle to late Miocene. The Miocene Climatic Optimum (MCO) in the early Miocene represents an interval of global warming which interrupted the overall Cenozoic cooling trend for more than 2 m.y. (Holbourn et al., 2015). In the late early to middle Miocene, there were global paleoclimatic and oceanic changes that resulted in the deposition of organic-carbon rich sediments into the marginal basins of California and around the Pacific margin (Ingle, 1981). During this time, there is a long-lasting positive carbon isotope trend called the Monterey Excursion, which started around 16.9 Ma and ended at ~ 13.5 Ma, about 400 kyr after a major expansion of the Antarctic ice sheet at 13.9 Ma (Vincent and Berger, 1985). Flower and Kennett (1993; 1994) selected the Naples Beach section as an excellent location to test the synchronicity between the organic carbon-rich deposition within the Monterey Formation and the positive δ13C trend of the Monterey Excursion. Located along coastline at the mouth of Dos Pueblos Canyon about 15 km west of Santa Barbara, the Naples Beach exposure contains the entire interval of the Monterey formation, (age range of ~ 17.85 to 7.5 Ma), only interrupted by erosional breaks at Dos Pueblos Canyon (~ 30 m of section not exposed) and in a condensed section. This section has well preserved benthic foraminiferal assemblages, limited diagenetic effects and age control from benthic foraminifera, calcareous nannofossils, diatoms, magnetostratigraphy, and strontium isotope stratigraphy. In the deep-sea record, high resolution time series spanning this critical climatic transition are limited because deep sea sedimentary sequences have been strongly affected by carbonate dissolution, and/or burial diagenesis and/or proven incomplete due to major changes in ocean circulation. The original oxygen and carbon isotopic studies used age models for the Miocene time interval that relied often on sparse biostratigraphic control and magnetostratigraphic data not directly calibrated to an astronomical timescale. In the deep-sea benthic foraminifera oxygen isotopic record, there is a sharp decrease (~ 1‰) in the benthic and bulk carbonate δ18O curves and onset of a pronounced (-0.6‰) negative shift in benthic and bulk carbonate δ13C ca. 16.9 Ma which represents the start of MCO, a rapid global warming and/or polar ice cap melting event (Holbourn et al., 2015). Rapid recovery in benthic and bulk carbonate δ13C after 16.7 Ma is associated with improved carbonate preservation. This recovery signals the onset of the first carbon isotope maximum within the Monterey Excursion (Vincent and Berger, 1985). The first δ13C maximum is >1.4‰ (CM1) is recorded at ~16.8 Ma (Holbourn et al., 2007) and represents the earliest δ13C maxima in Monterey Excursion. Based on the integration of orbital variations in climate proxy signals δ13C and δ18O and carbon from several ODP and IPOD sites in the central, southern and southeastern Pacific Ocean, there appears to be three distinct climate phases developed in the Miocene (Holbourn et al, 2007). Each phase is based on the oxygen isotopic record and its relationship to Miocene water mass characteristics. Phase 1 broadly corresponds to the early Miocene climate optimum (prior to 14.7 Ma) and consists of peak minimum values in benthic foraminiferal δ18O. This phase equates to minimum ice volume and to poor ventilation in the deep Pacific water masses. Phase 2 (~14.7 to 13.9 Ma) starts the long-term trend towards heavier δ18O values associated with punctuated climate cooling. This phase ends with rapid ice growth and global cooling at the onset of the last and most pronounced δ13C increase. During this transitional phase δ13C continues to show high amplitude 400 kyr variability and indicates improvement in Pacific deep-water ventilation. Phase 2 culminated with massive stepped increase in δ18O at 13.8 Ma marking entry into icehouse climatic conditions. Phase 3 continues after 13.9 Ma, when δ18O exhibits a long-term increasing trend and final entry into icehouse climatic conditions. This phase culminated with a substantial improvement in deep water ventilation and intensified production of southern source deep and intermediate waters. Initially in this phase, δ13C displays the highest values of whole middle Miocene at 13.8-13.6 Ma but subsequently shows reduced amplitude variations and long-term declining values (Holbourn et al., 2007) In the Naples Beach section, the Monterey Formation has generally been divided into the following members: a lower calcareous-siliceous marl, overlain by a carbonaceous marl, overlain by a transitional member, overlain by the upper calcareous to predominantly siliceous marl, and finally a siliceous member at the top of the interval (Issacs 1981, 1983). Benthic foraminiferal δ18O values at Naples Beach are low throughout the lower calcareous-siliceous member averaging between ~0.0 – 0.5‰ (Flower and Kennett, 1993, 1994). The first increase in δ18O occurs at the top of the first organic carbon-rich subunit within the lower Carbonaceous Marl member. There is a large permanent δ18O increase of ~ 1‰ found within the upper part of the Carbonaceous Marl member, which clearly precedes the phosphatic condensed interval. Above the condensed section, within the uppermost part of the Carbonaceous Marl member, the oxygen isotopic values reach ~1.5-2.0‰ (Flower and Kennett, 1994). Based on the age constraints at the time, Flower and Kennett (1994) proposed that the major increase in δ18O, was associated with the organic carbon-rich Organic Shale and is the main step of the well-known middle Miocene oxygen isotopic increase, now dated at 13.9 Ma. However, it now appears that this first large increase is associated with the first increase in δ18O that occurs at 14.7 Ma, at the base of Phase II interval (Holbourn et al, 2015). The main δ18O increase and end of the Monterey Excursion occurs within the Condensed Zone at approximately 13.9 – 10.2 Ma and represent the change into the icehouse climatic conditions. This is also the switch from calcareous dominated deposition to predominately siliceous deposition. Oxygen isotopic correlations of these organic carbon-rich intervals within the Naples Beach section of Monterey Formation suggest episodic increases in organic carbon deposition coincided with deep sea δ18O and δ13C maxima and synchronous global cooling events including the start of cooling at 14.7 Ma and the major middle Miocene cooling and expansion of the Antarctic ice sheet at 13.9 Ma. Improved correlation of Naples Beach section shows that phosphatic condensed interval (13.9 -10.2 Ma) clearly follows the major middle Miocene sea level drop Tor 1 (11.8 Ma). This would imply that there isn’t a relationship between major sea level falls and formation of this condensed section. However, another possibility is that it is related to water mass reorganization along the eastern Pacific margin and the tectonic evolution of the central -southern California margin in the mid to late Miocene represented by rotation of the Transverse continental block.