Geochemistry

X-ray diffraction data largely agree with those presented by Bilbey (1999), in that the Brushy Basin and Salt Wash Members of the Morrison Formation are primarily composed of quartz and calcite. The mineralogy of the CLDQ itself is distinguished from the surrounding Brushy Basin and nearby Salt Wash Members, as well as the mudstones at the same stratigraphic level as the CLDQ, by the presence of metal oxides (primarily litharge), sulfides (chalcopyrite and covellite) and barite. Furthermore, sulfide minerals (i.e., pyrite crystals) were found within IBFs from CLDQ (Figs. 7A, and 7B). This suite of minerals implies that the environment represented by the quarry was very likely reducing (Eby, 2004). High levels of decaying organic matter, i.e., an abundance of decaying dinosaurs, would utilize the available dissolved oxygen resulting in a reducing environment, especially given the ephemeral nature of the CLDQ pond. As discussed below, decaying dinosaurs may have been the source of the metals (Cu and Pb) needed to form metal oxides and sulfides.

The presence of calcite/barite nodules on the bones (Fig. 7C) further supports the hypothesis that the dinosaur remains were decaying in the CLDQ pond. Ligament and cartilage would be among the last of the organic matter to decay, and the highest concentration of dissolved organic matter would occur surrounding these tissues during late stage decay (Madsen Jr, 1976; Bilbey, 1999). The position of the calcite/barite nodules, most commonly found where ligaments and cartilage attached to bone (Madsen Jr, 1976), supports the hypothesis that they formed during decay. Furthermore, in subaqueous settings barite formation is associated with supersaturation of chemical microenvironments surrounding decaying organic matter (Paytan & Griffith, 2007).

Despite suggestions of an abundance of available organic matter at the CLDQ, evidence of scavenging is conspicuously rare, as previous authors have noted (e.g., Berner, 1968; Bilbey, 1998; Gates, 2005). However, hypereutrophic conditions due to the decomposition of dinosaur carcasses in the CLDQ deposit may also explain the rarity of scavenging traces on CLDQ bones. The presence of the calcite/barite nodules (Berner, 1968; Canfield & Raiswell, 1991) and sulfides (Wetzel, 2001) at the CLDQ suggests hypereutrophic freshwater conditions, which are commonly low diversity environments, especially with respect to vertebrates that would leave scavenging traces on bones (e.g., Heino, Virkkala & Toivonen, 2009). Hypereutrophic environments that have been recently disturbed, i.e., the ephemeral pond CLDQ likely represents, are particularly susceptible to low diversity (Moss et al., 2009).

X-ray fluorescence and x-ray diffraction data for the CLDQ show the quarry as enriched in heavy metals, e.g., Mo, As, U, Pb, relative to the rest of the local outcropping of the Morrison Formation. Some of these metals, Sr, Zn, Na and Mg, are known to easily replace Ca in biogenic apatite during diagenetic/post-depositional processes, such as interactions with groundwater (Trueman & Tuross, 2002; Goodwin et al., 2007). Furthermore, Morrison Formation bones are known to be enriched in U as a result of post-depositional and diagenetic processes, therefore elevated levels of U detected in CLDQ sediment and bones are not anomalous (Hubert et al., 1996). However, Gillette (1994) notes that U enrichment in dinosaur bone is most common in bones buried within the local water table. High U concentrations seen here help to support the hypothesis that the bones of CLDQ were deposited in an environment with a high water table, such as the shallow pond that CLDQ is suggested to represent (Gates, 2005).

Unfortunately, whereas studies of the geochemical compositions of fossil remains from bonebeds are common (e.g., Trueman & Benton, 1997; Trueman & Tuross, 2002; Rogers, Eberth & Fiorillo, 2010), similar studies focusing on the sediments from bonebeds are lacking in the literature. One possible origin for the heavy metals at CLDQ is accumulation through diagenetic processes. The high abundance of buried bone undergoing diagenetic dissolution may be responsible for the elevated levels of As. Elevated levels of As, Sr, Ce, Pb, and U have been noted in Dilophosaurus bones from the lower Jurassic Kayenta Formation, which is rich in iron oxides that can mobilize these metals, leading to the enrichment of bone material after burial (Goodwin et al., 2007). While this is a possible explanation for the origin of the metals detected at the CLDQ, it is worth noting, however, that even though Goodwin et al. (2007) found higher concentrations of As in bone than in the surrounding sediment (200–500 ppm in bone, 10–20 ppm in sediment), the opposite was seen at CLDQ (18–28 ppm in bone, 50 ppm in sediment). Two of the three sediment samples from MMQ contained more As than the bone from MMQ, following the pattern observed by Goodwin et al. (2007). Strong negative pair wise elemental correlations were observed for As and Fe in the CLDQ sediment samples, similar to what has been observed in material from the Kayenta Formation (Goodwin et al., 2007). This may reflect As desorption from iron oxides, given that Fe is low at the CLDQ and higher elsewhere in the Morrison Formation; the negative correlation between Fe and As at the CLDQ may be the result of desorption and absorption reactions during the dissolution and weathering of the large accumulation of bones. Bone dissolution would produce a high concentration of P, as observed via XRF, which can promote desorption of As (Goodwin et al., 2007). It is possible that similar processes resulted in the high concentrations of other metals in the sediments of CLDQ relative to concentrations seen in the rest of the Morrison Formation analyzed here, however these elements were not discussed by Goodwin et al. (2007). Elevated concentrations of metals seen in MMQ bone support the conclusion that diagenetic processes contributed to the heavy metal signature observed at CLDQ.

A second possibility for the origin of the elevated heavy metals at the CLDQ is bioaccumulation. Studies of modern grave soils suggest one potential source of heavy metals detected in bone and sediment of the CLDQ that are not necessarily explained by apatite diagenesis, i.e., Ni, Cu, Mo, As, Pb and W: the dinosaur carcasses themselves. Modern grave soils have long been seen as potential ecological hazards and sources of organic and inorganic pollutants (Aruomero & Afolabi, 2014). Whereas many studies of necrosols focus on burials with caskets which are not relevant to a Mesozoic bonebed (e.g., Ücisik & Rushbrook, 1998), some studies have focused on geochemistry of mass graves and primitive burials lacking caskets and burial goods (Kemerich et al., 2012; Amuno, 2013). Kemerich et al. (2012) utilized XRF to find elevated levels of Ba, Cu, Cr and Zn in the soil and groundwater associated with a mass grave in Brazil. Amuno (2013) found elevated levels of As, Cu, Cr, Pb and Zn in necrosols within and near to a mass grave site in Rwanda. Despite full soil development not being evident at the CLDQ, the deposit is an analogous accumulation of quickly buried vertebrate remains in fine grained sediment.

Even though it is highly unlikely that high concentrations of As, Cu, and Pb seen in CLDQ sediments indicate these metals occurred at toxic concentrations in the bodies of the dinosaurs which accumulated there (Goodwin et al., 2007), large numbers of carnivores decaying could lead to the accumulation of these metals in the CLDQ pond. Carnivores are especially likely to contribute heavy metals via trophic focusing of toxins as they are high-level consumers (e.g., Vijver et al., 2004; Gall, Boyd & Rajakaruna, 2015). Even though more extensive work is required to interpret the geochemical signal of the bones recovered from the CLDQ, the similarity of CLDQ sediment and preliminary bone geochemistry data, taken with the strong contrast between geochemistry of sediments from the CLDQ and from surrounding Morrison Formation sediments, implies a unique setting for the CLDQ assemblage.

The MMQ samples provide a significant comparison when considering bioaccumulation as a source of the metals observed at the CLDQ. Some elements found in CLDQ sediments and bone, specifically W, Cu, Ni and Cl, are found in higher concentrations in CLDQ materials than MMQ materials. However, Zr, Rb, V and K are found in higher concentrations at MMQ than at CLDQ.

Another possibility for the elevated presence of heavy metals found at the CLDQ is the dissolution of volcanic ash, which may have concentrated in the pond as it washed in during flood periods. Hubert et al. (1996) analyzed the chemical composition of “a large data base for silicic obsidians that proxies for the unknown composition of the altered silicic ashes in the Brushy Basin Member (page 537)”. The compositional data presented by Hubert et al. (1996) contrast significantly with the sediments of the CLDQ (Table 3). Given that the CLDQ sediment geochemistry does not match well with that of the proxy obsidians (Hubert et al., 1996), the metals present in the CLDQ are not likely sourced from local volcanic ashes emplaced during bone burial.

Element CLDQ Obsidian Cr 258–332 1–25 Ni 60–63 1–35 Pb 31–62 10–44 V 0 15–38 Rb 7–9 23–300 DOI: 10.7717/peerj.3368/table-3

Finally, the heavy metals present at CLDQ could be a result of past mining activities. This idea is unlikely for several reasons, however. First, the active quarry at CLDQ is covered by the North and South Butler buildings. Any metal-rich dust carried from mining operations would be more likely to settle on surfaces outside of the quarry which are exposed to air. Furthermore, the dense limestone cap over the bone-bearing layer at CLDQ makes it unlikely that the metals would have been transported vertically onto the bone-bearing layer as dust or in solution. Given that the analyzed sediment was exposed after the Butler buildings were constructed, the metals are not likely sourced from mining activities prior to the Butler buildings’ construction. Finally, given that metals are seen in high abundance in both the CLDQ and MMQ bonebeds, but not in any of the other 44 samples of Morrison Formation sediment analyzed, it is most likely that the metals are related to the presence of fossil bone, not recent mining activities.

Both XRD and XRF analyses, taken together, support the hypothesis that the CLDQ represents an ephemeral pond that became hypereutrophic as dinosaur carcasses decayed. The source of the calcite/barite nodules on the bones and sulfide minerals present in the quarry, but not found in other local Morrison sediments, appears to derive at least in part from the decay and dissolution of the dinosaurs themselves. Dinosaur decay could potentially have contributed to the heavy metal signature of CLDQ sediments as well.

Hypereutrophy can explain the near total lack of microvertebrate remains, (turtle, fish and crocodilian fossils typically associated with pond deposits), and near total lack of scavenging marks on CLDQ dinosaur bones. The typical freshwater fauna that would create microvertebrate remains and also scavenge on the carcasses (fish, turtles and crocodilians), would not have been able to tolerate hypereutrophic water conditions. Furthermore, as the carcasses rotted, the formation of calcareous soaps may have deterred extensive scavenging before leading to the formation of calcite/barite nodules on bones. While both diagenetic processes and a hypereutrophic water column are possible sources of the heavy metals found at the CLDQ, given the evidence in support of hypereutrophy of the CLDQ pond, diagenetic processes are not likely the primary cause of elevated concentrations for all of the metals seen here. Furthermore, if the accumulation of heavy metals seen at CLDQ is a result of apatite diagenesis associated with the large number of bones at the site, the site-specific taphonomy of the CLDQ could have contributed to the heavy metal signature, as metals which had bioaccumulated in a large number of top consumers remained in the CLDQ pond during decay and lithification. Geochemical analysis of the JONS site strengthens the interpretation of the CLDQ as a unique bone-bearing site within the Morrison Formation. Typical indicators of eutrophy (elevated levels of metals, sulfide minerals and calcite/barite nodules) are not present at JONS at the levels observed at the CLDQ. Furthermore, a typical freshwater microvertebrate assemblage of turtle and crocodilian remains are found at JONS. These data support the hypothesis that diagenesis is not the sole contributor of heavy metals at the CLDQ. JONS also contains large vertebrate bones, however the heavy metal signatures seen at the CLDQ are absent there. If organic remains are the primary source for metals, concentrations would not be expected to be as high at JONS as at the CLDQ, given the disparity in number of fossils found at each site. However, if post-burial diagenetic processes were the dominant source of heavy metals in the sediments of the CLDQ, some elevation in these metals would be expected at JONS.

Finally, analysis of MMQ sediment and bone provides meaningful contrast to that found at CLDQ and again highlights the uniqueness of CLDQ. The bones of MMQ show extensive evidence of biostratinomic alteration, implying the presence of scavengers. Although aquatic vertebrate remains are rare at MMQ, as at CLDQ owing to the ephemeral nature of the pond (Trujillo et al., 2014), they are more numerous at MMQ. Furthermore, the sediments of MMQ contain abundant carbonized plant fragments whereas the mudstone layer of CLDQ contains none. This is potentially due to differences in local vegetation during the time of deposition, but could also be a taphonomic effect. Bones recovered from MMQ are not associated with calcite/barite nodules. Rates of organic matter decay at MMQ must not have been high enough to form the calcareous soaps necessary for calcite formation. A second possibility is that freshwater input to the MMQ pond was high enough to flush the system, inhibiting the formation of such soaps. Taken together, these data imply that the MMQ pond was not hypereutrophic, whereas the CLDQ pond was. The differences in preservation of bone and plant material, the respective presence and absence of calcite/barite nodules, as well as differences in biostratinomy and microvertebrate fossil abundance between the two sites are best explained by variations in water chemistry: periodic hypereutrophic conditions at CLDQ, and an oligotrophic pond at MMQ.