A new breath of life for anoxia
Emmanuelle Pucéat
UMR CNRS 5561 Biogéosciences, Université de Bourgogne, 6 bd. Gabriel, 21000 Dijon, France
The middle of the Cretaceous (120–80 Ma) was one of the warmest named “Demerara bottom water mass.” Local exchange with aeolian or periods of the past 300 m.y., with tropical sea-surface temperatures well riverine particles weathered from the nearby Precambrian Guyana Shield over 30 °C (Pucéat et al., 2007; Forster et al., 2007a) and atmospheric CO would indeed have imprinted surface waters in the region with a very levels much higher than today. Therefore this period can give us crucial unradiogenic signature. This signature would then be carried by surface information on the mechanisms governing the climatic system in a con- waters as they sink to greater depth. As these intermediate waters derive text of extreme greenhouse conditions. Within this interval, major pertur- from a low-latitude area, they have to be very saline to be dense enough bations of the carbon cycle occurred around the Cenomanian-Turonian to sink in spite of their warm temperatures. Because the ε values of bot- boundary (93.5 Ma), evidenced by worldwide deposition of organic-rich tom waters at the Demerara Rise remain very unradiogenic except during (black) shales in the oceans, and a large positive carbon isotope excursion OAE2, MacLeod et al. suggest that the Demerara bottom water mass was refl ecting enhanced burial of 13C-depleted organic carbon. Such episodes present in this area during most of the Late Cretaceous.
of extensive organic-matter burial are known as oceanic anoxic events Production of warm, saline intermediate water has already been sug- (OAEs) and are thought to have an impact on global climate through atmo- gested in low- to mid-latitude evaporative seas for the Cretaceous and spheric CO draw-down (Forster et al., 2007b). Both increased primary Eocene period (Brass et al., 1982). This issue is, however, highly debated productivity leading to higher fl uxes of organic carbon to the seafl oor, and as most recent circulation model experiments point to a high-latitude source better preservation of organic matter due to anoxic conditions have been of deep waters, which would have been warmer than today due to green- invoked to explain enhanced organic-matter burial (Arthur et al., 1990). house forcing (Otto-Bliesner et al., 2002). In a recent study, Friedrich et al. Although sluggish ocean circulation is often called upon to explain wide- (2008) identify the existence of an interval of higher δ18O values in benthic spread ocean anoxia, we actually know very little about the global circula- foraminifera lasting ~1.5 m.y., prior to OAE2, at the Demerara Rise (Fig. 1). tion system during the Cretaceous. Apart from numerical simulations, few These authors interpret the higher δ18O values as evidence of an incursion of studies treat paleocirculation, and existing data on ocean structure remain warm and highly saline intermediate water at the Demerara Rise. Although very scarce for this period (Barrera et al., 1997; Pucéat et al., 2005; Soudry they both support the existence of such a water mass, the work of MacLeod et al., 2006). In this issue of Geology, MacLeod et al. (p. 811–814) present et al., based on an oceanic circulation tracer, contrasts to that of Friedrich new paleoceanographic data based on neodymium isotopes for the Late et al. (2008), as it points to a persistence of these intermediate waters at the Cretaceous period, and they track circulation changes in the southern North Demerara Rise during most of the Late Cretaceous. Yet, if ε values remain Atlantic across the oceanic anoxic event of the Cenomanian-Turonian consistently low except during OAE2, which argues in favor of a dominant (OAE2), one of the most prominent OAEs.
local source for the Demerara bottom water, it is interesting to note that Seawater neodymium isotopic ratios (represented by ε (0) = moderate variations in the ε record occur within this very unradiogenic ] – 1} × 104, and expressed in ε units; range prior to OAE2 (Fig. 1). Could these fl uctuations refl ect variations in CHUR is the chondritic uniform reservoir) are a good tracer of oceanic cir-culation because Nd has a short residence time (500 yr; Tachikawa et al., 2003) relative to oceanic mixing (~1500 yr; Broeker et al., 1960), and because the relative contributions of Nd from ancient continental-versus - young volcanogenic materials differ in the various basins. At present , the unradiogenic signature of North Atlantic Deep Water (ε = −13.5) derives from the contribution of Nd from old continental rocks such as those surrounding Baffi n Bay and the Labrador Sea (Stordal and Wasserburg, 1986). By contrast, the Pacifi c Ocean has a more radiogenic composition (ε = 0 to −5) derived from the weathering of island arc material (Piepgras Using the Nd isotope composition of fi sh debris, MacLeod et al. reconstructed the Late Cretaceous ε evolution of bottom seawater at the Demerara Rise (~10°N during the Late Cretaceous). Given the paleodepths of the studied sites (>1000 m for the deepest Ocean Drilling Program [ODP] site 1258), these waters can be defi ned as intermediate water masses. The fi rst remarkable result of this work is the very un radio- genic signature of these waters (−14 to −16.5) during most of the Late Cre- taceous. These values are the lowest reported for Cretaceous bathyal ocean sites, and are very close to ε values of the Davis Strait seawater, at the mouth of Baffi n Bay (typically −15 to −16 ε units; Stordal and Wasser burg, 1986). Although this feature cannot be uniquely interpreted yet, warm bot- Figure 1. Organic matter δ13C (δ13C , black squares; Friedrich et al.,
tom water temperatures reported in the southern North Atlantic (Friedrich 2008), benthic foraminifera δ18O (black crosses; Friedrich et al., 2008),
ε (black circles; MacLeod et al., 2008), and sea-surface tem-
et al., 2008) and the similarity of the Nd signature at three ODP sites sepa- peratures (SST; Forster et al., 2007a) as a function of core depth
rated by over 1000 m of depth at the Demerara Rise have logically led (mcd—meter composite depth) at Ocean Drilling Program site 1258,
MacLeod et al. to propose the existence of a locally derived water mass, Demerara Rise.
2008 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org.
GEOLOGY , October 2008; v. 36; no. 10; p. 831–832; doi: 10.1130/focus102008.1.
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Source: http://emmanuelle.puceat.free.fr/doc/Puceat08.pdf

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