6.1. Euxinia and Aliasing
 The model of Algeo and Lyons  works only where euxinic conditions exist(ed) in the water column: such euxinia is well documented for the Lower Toarcian black shales of NW Europe. The “degree of pyritization” (DOP) [Raiswell and Berner, 1985] through both the exaratum Subzone in Yorkshire and the Posidonia shale of Germany was shown by those authors to be around 0.85, a value attesting to long-term euxinic conditions in both basins. Similar values (0.8 to 0.9) for DOP were reported for the exaratum Subzone of Yorkshire by Pearce et al. . Wignall et al.  document anoxia and euxinia through the upper semicelatum Subzone and lower exaratum Subzone in Yorkshire on the basis of the size of pyrite framboids, and DOP. Our values of TS exceed 2%, and values of DOP-T (Figure 2) exceed 0.5, in the interval from the mid-semicelatum Subzone to the mid-falciferum Subzone. Such values confirm the general euxinic nature of the interval. In the upper falciferum Subzone values are less than 0.4, suggesting euxinic conditions no longer affected the water column, but they recover to between 0.4 and 0.5 above that level. Exceptionally low DOP-T is seen at the levels of two thin sideritic mudstones, beds 46 and 50.
 In Germany, Röhl et al.  showed that euxinia was prevalent throughout deposition of the Posidonia shale from the mid-semicelatum Subzone into the upper bifrons Zone. Photic zone euxinia was also documented by Schouten et al.  to be present at each of the seven levels that were examined through the same stratigraphic interval, but was shown to be absent at three lower stratigraphic levels. Schwark and Frimmel  broadly confirmed those findings using a higher sample density.
 Despite the prevalence of long-term euxinia over a substantial part of the time represented by the section in the Cleveland Basin, the presence of fossil “event communities” at uncommon, discrete, horizons within the sections in Yorkshire and the Posidonia shale of Germany [Howarth, 1962; Röhl et al., 2001] show that conditions were occasionally oxygenic, if only for periods of a few weeks to a few years at a time. The geochemical signal of oxygenation is aliased because sampling has not been undertaken (even by Pearce et al.  or Röhl et al. ) at a resolution high enough to capture the short periods when euxinia was interrupted and life flourished, briefly, in the Toarcian seas of NW Europe. We discuss this below.
 The palaeobiology of the Yorkshire sections has been presented by Little and Benton , Little , and Harries and Little . These authors note that organisms that inhabited the upper water column suffered little from these extinction events, and that epifaunal bivalves were also largely unaffected. The last work recognized the latest Pliensbachian and the earliest Toarcian (tenuicostatum Zone) as an interval in which extinction was most pronounced at four levels, the last occurring at the end of the interval (top of bed 32, i.e., top of the Grey Shale Member, Figure 6). During the time of deposition of the upper part of the semicelatum Subzone, species diversity and richness declined to low levels and remained low until the lower part of the commune Subzone (bifrons Zone). Above bed 50, in the commune Subzone, species diversity increased as populations recovered.
 While presenting a good synopsis of Early Toarcian biotas in Yorkshire, the work of Harries and Little [1999, Figure 6] nevertheless aliases the faunal data by joining adjacent sample points with a line. This disguises the fact that significant parts of the stratigraphic interval between the upper semicelatum Subzone to mid-commune Subzone; that is, the intervals between those levels where faunas were recorded, are devoid of macrofossils of any type. For the Posidonia shale of Germany this point of aliasing was noted by Röhl et al.  and Schmid-Röhl and Röhl . Documenting faunal and geochemical parameters at near centimeter-scale resolution through some 12 m of section at Dotternhausen in S Germany, they showed that the macrofossils are present at discrete horizons and represent “event communities,” i.e., opportunistic recolonizations of the seafloor that lasted only a few weeks to a few years [Röhl et al., 2001]. Such periods were rarely captured in geochemical parameters, such as C/S crossplots, probably because the sampling resolution, although exceptionally high, was still not high enough to do so. From faunal evidence, they noted around 30 such oxygenation events [Röhl et al., 2001, Figure 10] during the German equivalent of the United Kingdom's exaratum Subzone, with a higher density being recorded above that level.
 The pertinent point about such “event communities,” in which we include the uncommon belemnite horizons within the exaratum Subzone of Yorkshire (Figure 2), is that they record conditions only during the geologically brief moments when the fossils were alive [van de Schootbrugge et al., 2005; Wignall et al., 2005]. Their presence in Yorkshire and Germany attests to occasional brief periods when the general euxinic conditions were briefly interrupted and these periods have an importance far beyond their duration: into a long-term euxinic geochemical signal they interleave a short-term oxic geochemical signal (e.g., δ13C in belemnite calcite). The oxic signals were captured either during the brief periods of normal (open-ocean) conditions in the Cleveland Basin or, because belemnites were mobile, in the open (oxic) ocean outside the euxinic Cleveland Basin, into which the belemnites occasionally strayed to die [van de Schootbrugge et al., 2005; Wignall et al., 2005]. The chemical signatures of belemnites in the Cleveland Basin (or Posidonia shale) do not record the chemical signature of the euxinia generally prevalent within it, which is why they do not show the negative isotopic excursion shown by organic matter and (presumably?) nonmobile phytoplankton trapped in the basin and able to survive within the upper oxic interval of the water column.
6.3. Crossplot of Mo Versus TOC for the Cleveland Basin
 The Mo versus TOC crossplot (Figure 5) defines two data trends to which regression lines A and B have been fitted. Regression A has a slope of 0.5 and applies to samples collected from the upper half of the semicelatum Subzone and most of the exaratum Subzone (beds 31 to 38). This interval encompasses the putative Early Toarcian OAE, and the local negative excursion in δ13C of organic matter (Figure 2). The value of 0.5 is around 10 times less than that for Holocene sediments from the Black Sea (rsMo/TOC of 4.5 ± 1 [Algeo and Lyons, 2006]). It implies that almost complete restriction occurred during this interval, so we term it the interval of maximum restriction (IMR). The frequency of deepwater renewal in a restricted basin scales linearly with rsMo/TOC [Algeo and Lyons, 2006], so an rsMo/TOC of 0.5 implies a renewal frequency of deep water of 5 to 40 ka. Over a period of around 930 ka (Figure 2), renewal therefore occurred between 23 and 186 times. It is interesting that Röhl et al.  identified around 30 brief events of oxygenation during the equivalent time period in Germany, and ascribed them to the occurrence of tropical storms. The renewal frequency estimated by Röhl et al.  and here is much lower than is implied by the annual flushing model of Schwark and Frimmel  and Frimmel et al. , which, with today's concentration of marine Mo, would give rsMo/TOC around the 45 (cf. Saanich Inlet [Algeo and Lyons, 2006]).
 The infrequency of renewal implies that the concentration of Mo in the seawater of the basin was drawn down to concentrations well below those currently seen below 200 m in the Black Sea, where they are only 2–3% of that in open ocean water [Algeo and Lyons, 2006], so a mass balance of Mo is of interest here. For the 10 m of sediment in the IMR, the mean Mo concentration is 5.1 mg/kg (Table 1), a value that equates to 10 mg/cm2, using a sediment density of 2 g/cm2. This amount of Mo is contained within column of modern seawater (Mo 105 nM) that is 1000 m thick. This figure leaves little room for much replenishment of Mo during the IMR, were the Toarcian sea indeed 1000 m deep, so it implies that the Toarcian sea was very much less than 1000 m in depth, or that scavenging efficiency for Mo was nonlinear and declined at low concentrations of Mo in seawater.
 Within the IRM, sharp, albeit small, excursions in Re/Mo, Re/TOC, Mo/TOC, δ98Mo, and TOC coincide with the abrupt decreases in δ13C(org) (A–D in Figure 3). The Mo/TOC excursions are defined by either one datum (A and D) or two (B and C), where the sampling interval was 3 cm. As the major negative excursion in δ13C(org) occupies 10 m of strata from the mid-semicelatum Subzone to the basal falciferum Subzone (Figure 2), and endured over a period of 930 ± 40 ka [Suan et al., 2008], excursions A and D occupied <3 ka, while B and C occupied >3 ka but less than 9 ka. These durations are around one hundredth of the residence time of Mo in seawater today. Another subtlety of the excursions is that the middle pair (B, C) move to higher Mo/TOC, Re/TOC, and lower TOC, while the outer pair (A, D) move to lower Mo/TOC, Re/TOC, and higher TOC. At A and B, the decline in δ98Mo is briefly interrupted, while at C and D, the decline in δ98Mo is briefly strengthened, although these effects are small.
 These subtle changes must relate to changes in the rate of deepwater renewal. The importance of these excursions lies in their short duration, which provides good evidence that they are driven by changes in a reservoir of small size: they are consistent with control by a varying degree of restriction and inconsistent with any interpretation of the signals in terms of whole-ocean events, owing to the large mass of the ocean. The data defining the excursions plot on regressions of Mo versus TOC that have slopes of 0.4 to around 3.4, confirming that the variations in restriction they imply was small. It is not known whether these excursions coincide with oxygenic events identifiable by the presence of event faunas. The low frequency of water mass renewal accords with the faunal observations of only brief intervals of reoxygenation in this interval in Germany (the density of faunal event horizons has not been measured in the Cleveland Basin). Such intervals of oxygenation may represent the effects of 10 or 50 ka events, i.e., events of such severity that they occur only at those frequencies, that briefly overturned the water column. They are, perhaps, the tropical storms of Röhl et al. .
 Samples from above the IMR (beds 41 and above) fall along regression B (Figure 5), which has a slope of around 17. For modern sediments, this slope would imply a degree of restriction that is intermediate between that seen now for Framvaren Fjord (rsMo/TOC of 9 ± 2), where water mass renewal is around 50 to 130 years, and that seen in the Cariaco Basin (rsMo/TOC of 25 ± 2) where renewal times are around 50 to 100 years [Algeo and Lyons, 2006]. Restriction above the IMR was therefore less than it was in it. Excepting bed 49, both Mo concentrations (<8 ppm) and TOC concentrations (<3 %) are low in most strata above bed 45, where DOP-T is around 0.4 to 0.5, and TS around 2%. The Mo and TOC data nevertheless define a tight curvilinear regression (Figure 5) that suggests sulfidic sequestration of Mo was still happening. The data suggest that sediments above bed 45 were sulfidic, but that euxinic conditions had retreated to be close to the sediment-water interface in the Cleveland Basin, expanding briefly through bed 49.
 We speculate that a small part of the water column remained euxinic in much of this interval because of two factors: first, the recovery of faunal diversity did not begin until bed 50, in the lower part of the bifrons Zone, a matter which attests to the continued inhospitable nature of the environment until bifrons zone times. Second, the distribution of data along regression line B in Figure 5 may be interpreted roughly as reflecting the proportion of the overlying water column that was euxinic during times of moderate restriction. The position of a sample on the regression may therefore indicate the size of the reservoir available to supply Mo, and be a proxy for the depth of the chemocline. The faunal trends shown in Figure 6 therefore broadly reflect the geochemical trends through the entire composite section and confirm our interpretation of the profile of Mo/TOC as a proxy for degree of deepwater restriction.
6.4. Mo/TOC Profiles for Continental Europe
 Our Mo/TOC profiles for Germany and Switzerland differ between localities, the differences likely arise from differing degrees of restriction at differing times and places within the NW European Basin (loosely defined). In all four continental localities, values of Mo/TOC rise in, or below, the lower part of the tenuicostatum zone as restriction begins and intensifies, this is at a lower stratigraphic level than is found in The Cleveland Basin. At Dotternhausen, four thin units of organic-rich shale occur in the paltum and uppermost clevelandicum Szs. [Röhl et al., 2001] and it is tempting to equate those units with the four Sulfur Bands of Yorkshire. Across Germany and Switzerland, the onset of permanent restriction was therefore both earlier and less sudden than it was in the Cleveland Basin. This is no surprise; Wignall et al.  clearly documented diachroneity in the onset of black shale deposition across Europe. Although the application of the Mo/TOC proxy for Toarcian palaeoceanography in Germany and Switzerland would certainly benefit from more detailed sampling, and improved biostratigraphic calibration of existing Mo/TOC profiles, the existing data are significant in the sense that they corroborate the overall conclusions drawn from the detailed record of the Cleveland Basin in Yorkshire, and confirm the diachroneity of the onset of deepwater restriction across NW Europe in Early Toarcian time.