Antecedentes sobre las cotizaciones del mercado en la construcción de buques

The Phanerozoic is the final Eon in Earth history, extending from the end of the Neoproterozoic Era at 0.542 Ga, until now. It is divided into 3 Eras whose boundaries are palaeontologically defined and can alter in age with advances in dating and stratigraphic resolution. The Eras include the Palaeozoic (542 Ma to 251 Ma), the Mesozoic (251 Ma to 66.5 Ma) and the Cenozoic (66.5 Ma until present). The Phanerozoic is, of course, the age of animals and plants, and during this time, the Earth gained the character we are familiar with today. Whereas various geochemical lines of evidence, discussed above, suggest a late Neopro- terozoic rise in atmospheric oxygen concentrations, these levels did not likely reach modern values. Indeed, the Phanerozoic may have begun with oxygen in the range of only 20% of so of PAL. Lower-than-today early Phanerozoic atmos- pheric oxygen concentrations are consistent with some (Bergman et al., 2004), but not all (Berner, 2006; Berner and Canfield, 1989) biogeochemical carbon models. Lower early Phanerozoic oxygen levels, however, would also be consistent with Mo isotope evidence (Fig. 9.12), which suggests more removal of Mo into anoxic settings compared to today (Dahl et al., 2010). Both models and Mo isotopes therefore support a middle Proterozoic rise in oxygen levels in the middle Palaeo- zoic, to values approaching those of today (Bergman et al., 2004; Berner, 2006; Dahl et al., 2010).

Early Views on Phanerozoic Ocean Redox Structure. Our under-

standing of the history of Phanerozoic redox chemistry has evolved considerably over the last 30 years or so. In a classic early consideration of the subject, William

Berry and Pat Wilde (Berry and Wilde, 1978) analysed the time distribution of black shales, and concluded that anoxic ocean conditions were widespread during the early Palaeozoic, yielding to a greater oxygenation of the deep ocean in the later Paleozoic. This view is quite similar to that revealed recently from the molybdenum isotope record, as explained above. Berry and Wilde did not discuss the chemical nature of the anoxic waters, and it seems that they had something like a modern oxygen-minimum zone in mind. In one of the most elegant geochemical papers of which I am aware, Bob Berner and Rob Raiswell (Berner and Raiswell, 1983) explored with models, and by analogy with modern environments, the Phanerozoic history of the carbon and sulphur cycles. They forwarded many novel ideas in this paper, but of relevance here, they concluded that, in particular, euxinic conditions were widespread in the early Palaeozoic, diminishing in intensity in the Devonian. Sound familiar? Indeed, this work prompted Rob to develop tools, including the use of C/S ratios and DOP (see Sections 3.4 and 3.5) to specifically explore the redox nature of ancient seawater, where many specific instances of Palaeozoic euxinia were revealed (Raiswell and Al-Biatty, 1989; Raiswell and Berner, 1985; Raiswell et al., 1988; Raiswell et al., 2001). These ranked among the very first applications of palaeo-redox indicators to the geologic record.

Further Thoughts on Phanerozoic Redox History. Recent work by Tim

Lyons and his group has reinforced earlier discussions of early Palaeozoic euxinia by identifying widespread sulphidic conditions in association with the Cambrian SPICE (Steptoean Positive Carbon Isotope Excursion) anomaly (Gill et al., 2011). Paul Wignall has also highlighted a view linking the end-Permian mass extinc- tion to the mixing of sulphidic ocean deep water into shallow depths, poisoning animal communities in the sea and on the land (Wignall and Twitchett, 1996). Others have described a period of “superanoxia” starting in the end Permian and continuing into the early Triassic, where in deep sea sections from Southwest Japan and British Columbia, red clays and cherts give way to black shales (Isozaki, 1997). Lee Kump of Penn State has taken this idea further and has modelled the circumstances under which sulphide could have been released from the oceans to the atmosphere (Kump et al., 2005).

Euxinic intervals are also well-known and well-studied in the Cretaceous where black shales were deposited in a series of so-called ocean anoxic events (OAEs) (Jenkyns, 1980, 2010). Black shale deposition is mainly known from the proto-north and proto-south Atlantic, where, especially in the proto-north Atlantic, circulation may have been quite restricted. These intervals of black shale deposition have long been linked to euxinic conditions, and this is supported by the presence of biomarkers for sulphide-utilising anoxygenic phototrophic bacteria in black shale intervals (Damste and Koster, 1998; Kuypers et al., 2002) together with enrichments in redox sensitive trace metals, including Mo (Kuypers

Fe speciation protocols have recently been applied by Simon Poulton and his group to further explore the nature of bottom water chemistry during the 85 Ma Coniacian-Santonian Ocean Anoxic Event (OAE 3) (März et al., 2008) in the Demerara rise of the semi-restricted proto-North Atlantic. The Fe speciation data are a bit noisy, but when combined with the distribution of other elements, in particular P, V and Cd, the picture emerges of euxinic conditions alternating with non-sulphidic, possibly ferruginous conditions. While the sulphidic condi- tions dominate, the return to ferruginous conditions reflect periodically limited sulphide production in the water column. This could have resulted from periodic draw-down in sulphate concentrations due to, perhaps, changes in basin restric- tion, or to changes in the depth of the chemocline in the basin (März et al. 2008). The expanded application of Fe speciation protocols have revealed further instances of Phanerozoic ferruginous conditions. In particular, we uncovered evidence for ferruginous waters in the earliest Cambrian of the Yangtze Platform (Fig. 9.15) (Canfield et al., 2008). So clearly, some ferruginous water column condi- tions also accompanied the early age of animals in the Phanerozoic. In other work, my PhD student Emma Hammarlund found an interval of ferruginous conditions associated with the deposition of the 505 Ma Wheeler shale (Fig. 9.16) (Hammarlund, 2007), a site of soft-bodied fossil preservation contemporaneous with the famous Burgess Shale. Therefore, ferruginous conditions may have been more widely distributed in the Phanerozoic than we realise. As the Fe speciation techniques commonly used in Precambrian rocks are further applied to Phanerozoic sections we will better understand the balance between euxinic and ferruginous conditions through the Phanerozoic.

Why so Much Sulphide? Still, the available evidence suggests that when

anoxic, Phanerozoic waters tended towards euxinic conditions. This is in stark contrast to the preceding Neoproterozoic, where ferruginous conditions seem to have dominated. Why should this be so? We have some difficult explaining to do.

I will not pretend to solve this problem here, but I will offer some sugges- tions. Going back to the basics: ferruginous conditions dominate when the flux of highly reactive iron exceeds the delivery flux of sulphide (through sulphate reduction) in the 1 to 2 proportion needed to make pyrite (Fig. 9.3). In the Neopro- terozoic, ferruginous conditions dominated because sulphate reduction occurred in an environment where the sulphide produced was overwhelmed by the highly reactive iron flux. As described in Section 9.2, this preferentially occurs in areas of high sedimentation on the continental shelf receiving a high flux of highly reactive iron. In contrast, in the deep sea, the ratio between the potential supply of sulphate to drive sulphate reduction and highly reactive Fe delivery is more evenly balanced and can easily tip in favour of euxinic conditions if the waters go anoxic (Table 9.1). Logically then, one could conclude that in the Neoproterozoic, sulphate reduction tended to occur in ocean areas with an excess of highly reac- tive Fe, whereas the opposite was true in the Phanerozoic. I can think of several reasons why this could be true.

Fe (py) Fe (mag) Fe (ox) Fe (carb) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 100% 80% 0% 20% 40% 60% % Reactive Fe FeHR/Fe T Drum Mountain Swasey Spring

Figure 9.16 Fe speciation results from the Wheeler Formation (data from Hammarlund, 2007).

First, with lower levels of atmospheric oxygen, it is possible that the marine redox-cline was located rather far up onto the continental shelf. This would have exposed a greater proportion of the highly reactive iron flux to the oceans to anoxic marine waters encouraging ferruginous conditions. Indeed, described in Section 9.2, there is enough highly reactive iron delivered to the continental shelves to remove all of the sulphate flux to the oceans as pyrite, with plenty of highly reactive Fe to spare. This balance between the flux of highly reactive iron and rates of sulphide formation by sulphate reduction, however, will also depend on regional conditions of highly reactive iron delivery rates, and sulphate and organic carbon availability, as described in Section 9.6. These will change from place to place in the oceans allowing euxinic conditions in some locations, but not as a general condition. This was our model for the development of sulphidic conditions in the Proterozoic oceans as described above.

As oxygen concentrations increased at the end of the Precambrian, the marine chemocline deepens, removing the access of deeper anoxic waters to the excess highly reactive Fe on the shelf (except as a much smaller flux of pore- water Fe lost from the sediments to the over lying oxic waters; see Section 4.5). Increased oxygenation would have had another effect. The associated reduction in volume of anoxic waters (as suggested by Mo isotopes) would have reduced the anoxic sink for sulphide, allowing for more sulphate to be reduced to sulphide in the smaller volume of anoxic waters remaining. This happens today. There is so much sulphate available in seawater that the small amounts of modern-day anoxic water have a near limitless supply of sulphate through exchange with the large marine reservoir.

Another contributing factor may have been the evolution of animal grazers. Graham Logan and colleagues cleverly deduced some years ago that the pack- aging of organic matter in animal faecal material (as well as skeletonised algal hard parts somewhat later) may have accelerated the transport of metabolisable organic matter to the ocean depths and by doing so, reorganised biogeochemical cycles (Logan et al., 1995). Borrowing this idea, it seems plausible that animal evolution could have enhanced the efficiency of organic matter transport out of the upper photic zone for rapid delivery to depth into outer shelf and conti- nental slope environments. Here, the sulphide produced by the decomposition of this elevated flux of organic matter by sulphate reduction may have been able outpace the lower availability of highly reactive Fe in this setting. Indeed, Berry and Wilde (Berry and Wilde, 1978) highlight that Palaeozoic black shale deposits develop primarily on continental slopes. Futhermore, many of the examples of Phanerozoic euxinia described above are also from deep water settings.

Yet another biological factor may have been the evolution of bioturbating animals. Bioturbation has the effect of mixing the sediment, encouraging sulphide oxidation and limiting the amount of pyrite (Section 3.4 and Berner and Westrich, 1985; Canfield and Farquhar, 2009) that can form. If less sulphur is buried in continental shelf sediments, more sulphate becomes available for sulphate reduc- tion elsewhere, such as in euxinic environments. This would further enhance the availability of sulphate to a more limited volume of anoxic waters as generated through elevated oxygen levels.

Therefore, it seems that a number of factors have combined to contribute to the apparent dominance of anoxic euxinic conditions (as opposed to ferrugi- nous conditions) through the Phanerozoic. These factors include aspects of both atmospheric oxygenation and biological evolution. Whether there are links between these two is a frontier area of research. It is possible, though, that these factors could have combined in different magnitudes as the intensity of bioturba- tion intensified through time and as atmospheric oxygen levels increased with a different history. More intense utilisation of Fe speciation protocols should provide the data base by which to elucidate the history of Phanerozoic ocean chemistry, the basis by which these ideas and others can be tested.

Furthermore, these ideas need to be validated with careful biogeochemical modelling.

9.8 Summary

Throughout Earth’s history, the redox chemistry of the oceans has been controlled by the interplay, mainly, between the cycles of oxygen, sulphur and iron. The evolution of this interplay is shown in Figure 9.17. Early in the Archean, there is little evidence for any pervasive oxygenation of the Earth surface, and if present, oxygen was presumably low enough in concentration to hinder the weathering of sulphide on land to sulphate. The oceans were anoxic (save possibly for some oxygen-containing oases if cyanobacteria were present) and dominated by Fe cycling (field “a” in Fig. 9.17). Many geochemical indicators point to the periodic late Archean oxygenation of the Earth surface. Ocean redox seemed to respond with the occasional establishment of euxinic conditions. Still, euxinia seemed to have been rare and ferruginous conditions dominated (field “b” in Fig. 9.17). During the Great Oxidation Event, atmospheric oxygen increased, promoting effective weathering of sulphide to sulphate on land. We have few obser- vations on the nature of ocean redox associated with and immediately after this event, but a reduction in BIF deposition is indicated, promoted, most likely, by an increase in the sequestration of highly reactive Fe by the sulphide produced from elevated rates of sulphate reduction.

There are numerous indica- tions of euxinic intervals after the deposition of the last major episode of BIF at 1.84 Ga. The emerging view, however, is that euxinic conditions were concentrated either in restricted basins or as wedges of sulphidic water extending from the continental shelf, but giving way to ferruginous deep water conditions further offshore. Although ferruginous waters may have dominated the oceans by volume, euxinic conditions acted as an impor- tant sink for highly reactive Fe (and other redox-sensitive trace metals), limiting the concentrations of Fe(II)

Figure 9.17 Cartoon illustrating how the viability of sulphide and Fe availability through time has influenced ocean chemistry. For further details, see text, especially Section 9.8.

in the ferruginous waters, and thereby limiting the deposition of BIF. It is possible that oxygenated deep waters were also present during the middle Proterozoic (field “c” in Fig. 9.17).

It is an open question whether Neoproterozoic chemistry was much different from the preceding middle Proterozoic (field “c” in Fig. 9.17). Mo isotope evidence suggests that they may be similar, but the paucity of observed euxinic intervals in the Neoproterozoic may suggest that this time in Earth history was, indeed, more ferruginous than the time before. It is not completely clear why this should be so, but pyrite subduction through the middle Proterozoic could have decreased the surface inventory of sulphur, increasing the excess of highly reactive Fe over sulphide.

There is evidence for a late Neoproterozoic oxygenation of the surface envi- ronment, which likely reduced the overall extent of anoxia in the global ocean. Where anoxic, the Phanerozoic ocean was dominantly euxinic, in stark contrast to the apparently dominant ferruginous conditions of the Neoproterozoic (field “d” in Fig. 9.17). This is indeed a puzzle, but elevated oxygen levels may have increased the depth of the chemocline, restricting the access of highly reactive Fe to deeper anoxic waters. An expansion of oxic conditions also reduced the overall volume of anoxic waters, allowing a greater sulphate supply to drive sulphate reduction in the lower volume of anoxic water. Furthermore, the evolution of animals allowed rapid export of faecal material to depths in the ocean where the highly reactive Fe flux is limited. Therefore, rates of sulphate reduction may have been accelerated in highly reactive-Fe-poor depths of the ocean, tipping the balance towards euxinic conditions. The evolution of bioturbation by animals may have also limited sulphur removal as pyrite on continental shelves, increasing the availability of sulphate for sulphate reduction in anoxic areas of the ocean.