Neogene and Quaternary calcareous nannofossils from the Blake Ridge, Sites 994, 995, and 997

Okada, Hisatake

doi:10.2973/odp.proc.sr.164.232.2000

Cited by 32 publications

(22 citation statements)

References 30 publications

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“…This pattern of very rare Gephyrocapsa spp. (>4 µm) occurrences below the Pliocene/Pleistocene boundary is consistent with results from Site U1312 (see the "Site U1312" chapter) and reports of similar occurrences from the Blake Ridge, northwest Atlantic (Okada, 2000). Thus the FOs of G. oceanica and G. caribbeanica are placed at the base of consistent occurrences of these species.…”

Section: Site U1313supporting

confidence: 88%

Data report: calcareous nannofossils from upper Pliocene and Pleistocene, Expedition 306 Sites U1313 and U1314

Hagino¹,

Kulhanek²

2009

Proceedings of the IODP

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Integrated Ocean Drilling Program Expedition 306 recovered expanded Pliocene-Pleistocene sections at two sites in the North Atlantic. Site U1313 was drilled on the western flank of the Mid-Atlantic Ridge northwest of the Azores, whereas Site U1314 was drilled on the southern Gardar Drift. Calcareous nannofossils from both sites are abundant and moderately to well preserved in the upper Pliocene and Pleistocene. The assemblages are dominated by gephyrocapsids and reticulofenestrids. Biostratigraphic analysis of the upper 158 meters composite depth (mcd) of Site U1313 yields a complete succession of nannofossil datums spanning the last 3 m.y. Site U1314, which had higher sedimentation rates during the Pleistocene, spans the last 1 m.y. in the upper 96 mcd.

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Section: Site U1313supporting

confidence: 88%

Data report: calcareous nannofossils from upper Pliocene and Pleistocene, Expedition 306 Sites U1313 and U1314

Hagino¹,

Kulhanek²

2009

Proceedings of the IODP

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“…The sedimentation rate above reflector A (0.8 Ma) at the end of the Line 16 is about 22 cm/ky (180 m / 0.8 m.y. ), which is comparable to the Miocene sedimentation rate of 26 cm/ky on the ridge crest but much higher than the Quaternary rate of -6 cm/ky [Okada, 2000]. The apron was drilled at Sites Figure 11.…”

Section: The Dynamic Free Gas Systemmentioning

confidence: 68%

Seismic Studies of the Blake Ridge: Implications for Hydrate Distribution, Methane Expulsion, and Free Gas Dynamics

Holbrook

2013

Natural Gas Hydrates

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I summarize results of active-source seismic investigations, including singlechannel seismic reflection, ocean-bottom seismic refraction, and vertical seismic profiles, from the Blake Ridge, offshore South Carolina. The Blake Ridge is a highly dynamic system, in which methane gas is being generated today at depths exceeding 700 m and migrates on the sub-Ma time scale. Seismic velocities indicate that hydrate occupies 3^4% of total rock volume (5-7% of porosity) in a 250-m-thick zone on the crest of the ridge and only about 1-2% of total rock volume 7 km off the crest of the ridge. A significant proportion of methane resides beneath the stability zone, where free gas comprises 1-10% of total rock volume in a zone at least 250 m thick. The impedance contrast responsible for the BSR is caused principally by gas, not hydrate: Site 994 shows that hydrate is not a sufficient condition for generation of a BSR, while Sites 995 and 997 suggest that free gas is a necessary condition. Zones of low reflectance beneath the BSR demonstrate that amplitude "blanking" is not required to explain low reflectance above the BSR on the Blake Ridge. Gas migrates along permeability contrasts at stratal boundaries and can be trapped by faults and at the base of the hydrate stability zone. The Blake Ridge collapse structure, which shows strong circumstantial evidence for expulsion of large quantities of methane into the ocean/atmosphere system, does not appear to have been a single catastrophic event. Rather, the collapse appears to be a relatively long-lived feature filled in the last 2 my at deposition rates of up to 300 m/my. Buried features similar to the collapse beneath the ridge suggest that episodic gas release is a fundamental, recurring part of the hydrate cycle on the Blake Ridge.

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“…Production and melting of icebergs during that time released large quantities of ice rafted debris, which supported exceptional sedimentation rates ranging from 76 to 162 cm ka -1 (Blaise et al, 1990 (Okada, 2000).…”

mentioning

confidence: 84%

Methane sources in gas hydrate-bearing cold seeps: Evidence from radiocarbon and stable isotopes

Pohlman

Bauer²,

Canuel³

et al. 2009

Marine Chemistry

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ABSTRACT. Fossil methane from the large and dynamic marine gas hydrate reservoir has the potential to influence oceanic and atmospheric carbon pools. However, natural radiocarbon ( 14 C) measurements of gas hydrate methane have been extremely limited,and their use as a source and process indicator has not yet been systematically established. In this study, gas hydrate-bound and dissolved methane recovered from six geologically and geographically distinct high-gas-flux cold seeps was found to be 98 to 100% fossil based on its 14 C content. Given this prevalence of fossil methane and the small contribution of gas hydrate (<1%) to the present-day atmospheric methane flux, non-fossil contributions of gas hydrate methane to the atmosphere are not likely to be quantitatively significant. This conclusion is consistent with contemporary atmospheric methane budget calculations.In combination with G 13 C-and GD-methane measurements, we also determine the extent to which the low, but detectable, amounts of 14 C (~1-2 percent modern carbon, pMC) in methane from two cold seeps might reflect in situ production from near-seafloor sediment organic carbon (SOC). A 14 C mass balance approach using fossil methane and 14 C-enriched SOC suggests that as much as 8 to 29% of hydrate-associated methane carbon may originate from SOC contained within the upper 6 meters of sediment. These findings validate the assumption of a predominantly fossil carbon source for marine gas hydrate, but also indicate that structural gas hydrate from at least certain cold seeps contains a component of methane produced during decomposition of non-fossil organic matter in near-surface sediment.

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Neogene and Quaternary calcareous nannofossils from the Blake Ridge, Sites 994, 995, and 997

Cited by 32 publications

References 30 publications

Data report: calcareous nannofossils from upper Pliocene and Pleistocene, Expedition 306 Sites U1313 and U1314

Data report: calcareous nannofossils from upper Pliocene and Pleistocene, Expedition 306 Sites U1313 and U1314

Seismic Studies of the Blake Ridge: Implications for Hydrate Distribution, Methane Expulsion, and Free Gas Dynamics

Methane sources in gas hydrate-bearing cold seeps: Evidence from radiocarbon and stable isotopes

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