Untitled

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.
the intensity of intermediate water production in the southern North Atlantic REFERENCES CITED
prior to OAE2, which might then reconcile the benthic foraminifera δ18O Arthur, M.A., Jenkyns, H.C., Brumsack, H.J., and Schlanger, S.O., 1990, Stra- tigraphy, geochemistry and paleoceanography of organic carbon-rich Cre- record of Friedrich et al. (2008) with ε values? The decrease, with oscil- taceous sequences, in Ginsburg, R.N. and Beaudoin, B., eds., Cretaceous lation, of ~1.5 ε units in the 3 m.y. that precede OAE2 may indeed refl ect Resources, Events and Rhythms, NATO ASI Series C: Berlin, Springer- an increasing contribution of a warm, highly saline (high δ18O) and locally derived (low ε ) water mass. At any rate, more ε data from this interval Barrera, E., Savin, S.M., Thomas, E., and Jones, C.E., 1997, Evidence for are needed to further discuss this issue and the intriguing short-term 3 ε unit thermohaline-circulation reversals controlled by sea-level change in the latest Cretaceous: Geology, v. 25, p. 715–718, doi: 10.1130/0091-7613 positive excursion recorded just before the beginning of OAE2.
(1997)025<0715:EFTCRC>2.3.CO;2.
Nevertheless, these ε variations appear limited when compared to the Brass, G.W., Southam, J.R., and Peterson, W.H., 1982, Warm saline bottom water very large positive excursion of 8 ε units detected during OAE2. Again, as in the ancient ocean: Nature, v. 296, p. 620–623, doi: 10.1038/296620a0.
acknowledged by MacLeod et al., this excursion cannot be uniquely inter- Broeker, W.S., Gerard, R., Ewing, M., and Heezen, B.C., 1960, Natural radio- carbon in the Atlantic Ocean: Journal of Geophysical Research, v. 65, preted, and the infl uence of radiogenic Nd derived from Caribbean large p. 2903–2931, doi: 10.1029/JZ065i009p02903.
igneous province eruptions cannot be totally excluded. A signifi cant con- Forster, A., Schouten, S., Baas, M., and Sinninghe Damsté, J.S., 2007a, tribution of Nd from hydrothermal sources would, however, require an Nd Mid-Cretaceous (Albian-Santonian) sea surface temperature record of budget in the oceans markedly different than today’s. Yet, if the Demerara the tropical Atlantic Ocean: Geology, v. 35, p. 919–922, doi: 10.1130/ bottom water mass model is correct, as MacLeod et al. suggest, evolution Forster, A., Schouten, S., Moriya, K., Wilson, P.A., and Sinninghe Damsté, J.S., of intermediate water Nd isotope composition toward less negative values 2007b, Tropical warming and intermittent cooling during the Cenomanian/ can be interpreted as a temporary interruption of the sinking of intermedi- Turonian Oceanic Anoxic Event (OAE 2): Sea surface temperature records ate water in the southern North Atlantic. If waters from Tethyan and other from the equatorial Atlantic: Paleoceanography, v. 22, p. PA1219, doi: North Atlantic sites had a more radiogenic composition during the Mid- Friedrich, O., Erbacher, J., Moriya, K., Wilson, P.A., and Kuhnert, H., 2008, Cretaceous (typically −10/−5 ε units), we do not yet know the isotopic Warm saline intermediate waters in the Cretaceous tropical Atlantic Ocean: signature of South Atlantic waters during this period. Discussions about Nature Geoscience, v. 1, p. 453–457, doi: 10.1038/ngeo217.
possible sources of intermediate waters replacing the Demerara bottom MacLeod, K.G., Martin, E.E., and Blair, S.W., 2008, Nd isotopic excursion across water mass during OAE2 would therefore remain very speculative, given Cretaceous ocean anoxic event 2 (Cenomanian-Turonian) in the tropical the present state of knowledge about the Nd isotope composition of Cre- North Atlantic: Geology, v. 36, p. 811–814, doi: 10.1130/G24999A.1.
Otto-Bliesner, B.L., Brady, E.C., and Shields, C., 2002, Late Cretaceous ocean: taceous water masses. Nevertheless, the work of MacLeod et al. points to Coupled simulations with the National Center for Atmospheric Research changes in water mass pathways, and the location of deep water produc- Climate System Model: Journal of Geophysical Research, v. 107, doi: tion sites during OAE2, even if more ε data from other bathyal sites are needed to clarify the exact nature and spatial extent of these changes. There- Piepgras, D.J., and Jacobsen, S.B., 1988, The isotopic composition of neo- dymium in the North Pacifi c: Geochimica et Cosmochimica Acta, v. 52, fore, these results have important implications for our understanding of the p. 1373–1381, doi: 10.1016/0016-7037(88)90208-6.
processes that lead to extensive black shale deposition. If it is still diffi cult to Pucéat, E., Lécuyer, C., and Reisberg, L., 2005, Neodymium isotopic evolu- defi ne whether an ε increase precedes or follows the beginning of the δ13C tion of the Western Tethyan seawater throughout the Cretaceous: Earth excursion marking OAE2, the decrease of ε values clearly begins before and Planetary Science Letters, v. 236, p. 705–720, doi: 10.1016/j.epsl.
the return of δ13C to pre-excursion values. This could indicate a causal role Pucéat, E., Lécuyer, C., Donnadieu, Y., Naveau, P., Cappetta, H., Ramstein, G., of ocean circulation in worldwide black shale formation, either through Huber, B.T., and Kriwet, J., 2007, Fish tooth δ18O revising Late Creta- bottom water oxygenation or through nutrient concentrations at the depth ceous meridional upper ocean water temperature gradients: Geology, v. 35, tapped by upwellings. The exact nature of this role, and its link with rapid p. 107–110, doi: 10.1130/G23103A.1.
climate variations inferred from sea surface temperature changes (Forster Soudry, D., Glenn, C.R., Nathan, Y., Segal, I., and VonderHaar, D.L., 2006, Evo- lution of Tethyan phosphogenesis along the northern edges of the Arabian- et al., 2007b; Fig. 1), and with large-scale magmatic activity (Turgeon and African shield during the Cretaceous-Eocene as deduced from temporal Creaser, 2008), which may have affected oceanic patterns by displacement variations of Ca and Nd isotopes and rates of P accumulation: Earth-Science of seawater during plateau formation, still remain to be determined.
Reviews, v. 78, p. 27–57, doi: 10.1016/j.earscirev.2006.03.005.
Many of the uncertainties concerning the interpretation of ε records Stordal, M.C., and Wasserburg, G.J., 1986, Neodymium isotopic study of of specifi c water masses come from the scarcity of existing ε data rele- Baffi n bay water: Sources of REE from very old terranes: Earth and Planetary Science Letters, v. 77, p. 259–272, doi: 10.1016/0012-821X vant to the Cretaceous oceans. The work of MacLeod et al. provides a valuable ε record of intermediate water masses in the tropical North Tachikawa, K., Athias, V., and Jeandel, C., 2003, Neodymium budget in the mod- Atlantic during the Late Cretaceous. It is essential that the Nd isotope ern ocean and paleo-oceanographic implications: Journal of Geophysical signatures of potential sites of deep water formation be characterized, in Research, v. 108, p. 3254, doi: 10.1029/1999JC000285.
Turgeon, S.C., and Creaser, R.A., 2008, Cretaceous oceanic anoxic event 2 trig- order to be able to track the origin of deep water masses bathing the Creta- gered by a massive magmatic episode: Nature, v. 454, p. 323–326, doi: ceous oceans. Hopefully the work of MacLeod et al. will spur more effort to generate such Nd isotope records, which will help to constrain ocean structure and circulation patterns during the Cretaceous.

Source: http://emmanuelle.puceat.free.fr/doc/Puceat08.pdf