Bone tissue preservation in seawater environment: a preliminary comparative analysis of bones with different post-mortem histories through anthropological and radiological perspectives

Bone taphonomy has always fascinated forensic scientists for many reasons, some more valid perhaps than others: at times it seems to have strong correlations, but not always, with postmortem interval (PMI), in other cases it seems to relate to the burial or deposition site of the body, and sometimes macroscopic taphonomy may be envisaged as reflecting the potential the bone has for providing microscopic, biomolecular, chemical information. Many times, however, research has proven that these correlations may not be so [see, for example, 1‐5].

Since the nineteenth century, several studies have been carried out to better understand the diagenetic processes which affected human remains coming from different environments [e.g. 6‐16]. Over the years, several mechanisms responsible for bone deterioration have been described, including biological and chemical degradation of the collagen fraction and dissolution and recrystallization phases of the mineral component [9, 12, 13, 16, 17]. A mutual protection was proposed as responsible for bone stability during the post-mortem period between these two fractions (the collagen and the mineral fractions), and its breaking would result in a rapid degradation of bone tissue [18]. Among the different mechanisms, the microbial degradation of bone has been one of the most investigated topics. In 1981, Hackett was the first to describe and characterise the forms of foci and tunnels [8], with several scientists after him proceeding with the study of bone tissue microstructure with a special focus on microbial activity in bones coming from different environments. The possible patterns of microbial activity that can be found in different contexts have been described, especially those related to outdoor and water environments, reporting the time at which the first signs were visible within bone tissue [see, for example, 15, 19‐23]. However, despite the considerable number of studies, little is still known about marine bone taphonomy, especially for samples coming from deep sea and with a short PMI.

Considering the difficulties in setting experimental investigations in marine environments, information generally comes from forensic cases [24‐27], which are focused on body preservation and macroscopic appearance of the recovered skeletal remains. Studies on bioerosion were also carried out, but analyses were limited to experimental substrates (e.g. limestone and PVC plates) [28‐31]. Other bioerosion studies were carried out on pigs, but without analysing bone tissue when skeletonization was reached [32‐34]. Considering bone histological investigations, data were recorded especially from archaeological material (tenth to sixteenth centuries) [20, 23, 35, 36], and only in few cases from recent human skeletal remains [19].

In the last years, methods other than microscopy were used in taphonomical research, such as chemical and radiological analyses. Among these, the analysis of bone mineral density (BMD) has been included in many studies. In particular, previous investigations focused on the changes in bone mineral content at several portions on human bones by different techniques (single photon absorptiometry, peripheral quantitative computed tomography (pQCT), and dual energy X-ray densitometry) [37‐39], suggesting a possible relation between the density of bone elements or portions and their degree of environmental preservation [38, 39]. However, no studies have investigated BMD on skeletal remains coming from marine environments, nor have they evaluated whether variations in mineral content exist between different environments. In addition, no comparisons were carried out so far among bones recovered from different depositional scenarios with a multidisciplinary approach (medico-legal, anthropological and radiological).

Collecting more data on bone tissue preservation of contemporary samples would provide important insights on the diagenetic trajectory which affected skeletal remains, as well as regarding the mechanisms involved in the preservation/degradation of the bone components (collagen and mineral phase). This is even more important from environments for which little information is available, such as the sea [27, 40]. This information would have also important implications in laboratory analyses that involve the study of the bone tissue and its molecular components, such as age estimation, pathological diagnosis and genetic studies. In addition, comparisons with other samples with a different post-mortem history would allow identification of the peculiar traits which can differentiate the different burials and environmental changes acting as a fingerprinting of the diagenetic events.

The present pilot study aims to compare the state of preservation of bones recovered in seawater with bones coming from different environments (outdoor and fresh water environments and buried in coffin) from a macroscopic, microscopic and bone densitometric perspectives. This is the first type of comparative study in this sense to our knowledge.

The present pilot study provided a preliminary insight on the state of preservation of bodies and bone samples from a marine environment, and specifically from great depths (400 m) and with short PMI (4–14 months). This was done from several points of view (macroscopic, microscopic and densitometric), by comparing data of similar samples collected from individuals which were buried in coffins, as well as recovered in outdoor or fresh water environments.

Macroscopic and microscopic methods were selected as they are the reference methods in bone taphonomy research and the most used to describe bone tissue preservation [see 8, 21, 26, 45‐48]. However, to collect data mainly on the mineral component, dual energy X-ray absorptiometry was selected as non-destructive method able to measure the mineral content and to provide additional data before carrying out any further destructive analyses.

In literature, the few studies focusing on marine taphonomy are limited to body decomposition [24‐27] and to experimental investigations on pigs or bioerosion [28‐34]. However, not many analyses were carried out on the skeletal tissue in order to evaluate the cellular and molecular changes which occur in marine contexts in contemporary samples.

Our results showed an overall good preservation of bone tissue of the sea water samples from both the macroscopic and microscopic point of view, regardless of the state of decomposition of the body. Only three samples displayed cortical bone erosion possibly due to fauna activity in agreement with the observations performed by Pokines et al. [26] and the experiments carried out using pig carcasses by Anderson and Bell [32‐34]. Despite bone loss, the samples were characterised by well-preserved tissue, and no microorganism invasion in the form of tunnels such as those described in several studies [8, 19, 20, 23, 35] was detected. The absence of such microbial activity could be explained with an insufficient time since death to allow microorganisms’ invasion and growth. The observations performed by Yoshino and colleagues [19] supported this theory. In fact, they observed microscopically the first signs of microorganism activity in recent human remains recovered from the marine context 4–5 years after death. Similarly, even if using a different substrate (limestone) and focusing on macrobioerosion, Färber and colleagues [31] (experimental condition: close to Greek cliff, water depths between 3 and 17 m) observed no activity within the first year of exposure, thus identifying an increasing diversity and intensity of bioerosion agents over time. However, time could not be the only variable. In fact, since some archaeological samples (tenth to sixteenth centuries) showed bone matrix characteristics similar to those observed in fresh bones [20, 35, 36], environmental conditions have to be also included among the factors influencing bone tissue preservation. Several studies highlighted the effects of environmental parameters such as temperature, light, depth, dissolved oxygen on faunal compositions and diversity related to body preservation [24, 25, 28‐34]. Generally, a decrease in life and diversity has been observed with increasing depths and differences were recorded in different seasons, with the lowest activity and diversity documented in winter at the deepest depth considered (250 m) [30]. In their study Arnaud and colleagues [35] explained the good preservation of the observed bone matrix as the result of a wet salting procedure of the sample during the post-mortem period. In such a case, the salt contained in the sea water acts similarly to the salt-fish conservation, as it penetrates into the tissue with a better bone structure preservation.

In order to verify the presence of differences among bones coming from different environments, seawater samples were compared with other cases coming from outdoor and fresh water environments and cases buried in a coffin. Previous studies in literature compared samples from different post depositional contexts [15, 19, 21, 23], but none of them investigated taphonomic parameters such as the general body preservation, the macroscopic and microscopic tissue appearance and the bone mineral content.

When comparing samples from the various analysed contexts (sea water, fresh water, outdoor and burial in coffin), no differences were observed in terms of bone macroscopic and microscopic preservation. Specifically, the microscopic aspect proved similar high OHI scores in both undecalcified and decalcified thin sections, suggesting a good preservation of both organic and inorganic components. Only few samples from individuals buried in coffins showed limited microorganism invasion (10–30%). Regardless the bone element analysed, the present results are in agreement with those reported in literature: limited microorganism invasions were found at mid-diaphysis in femurs and tibiae of buried individuals by Caruso et al. [48], and the appearance of first signs of microbial activity within the bone tissue was found near the surgical neck of humeri after 15 years of decomposition on the ground by Yoshino et al. [19]. The similar preservation observed suggested that regardless the region of sampling, bone diaphysis shows a comparable bone tissue preservation. Differences were instead highlighted on the state of preservation of the body. In fact, an advanced decomposition was found mainly in fresh water and outdoor samples, while a greater amount of seawater and buried cases were partially or completely skeletonised. The complete skeletonization of samples from buried corpses could be ascribable mainly to the longer time since death (PMI: 20 years); for this reason, a higher number of samples coming from a similar environment and with a shorter PMI (1 year) are needed to perform additional comparisons. On the contrary, samples recovered from the sea showed results at times slightly opposite to what has been reported in literature: Ellingham et al. [27] discoursed about a decomposition process slower than that occurring on ground for corpses decomposing in seawater contexts because of the lower temperatures, the absence of insects and the salinity which decreases bacterial activities. However, as highlighted by Anderson and Bell [34], at depths of 100–300 m remains are mainly scavenged than decomposed, and this data might justify why the majority of bodies proved partially or completely skeletonised in our sample. In this regard, faunal composition and necrophagous communities are the main factors influencing the speed of skeletonization of a body [24]. Examples of skeletonised and disarticulated bodies with a short PMI (few months) have been described in literature [24, 32].

Even if slight differences were highlighted among the groups, body preservation seemed not to influence the macroscopic and microscopic appearance of each sample. In fact, even in case of complete skeletonization of the body, macroscopic and microscopic bone appearance was similar in all the contexts. Similarly, no relation exists between macroscopic and microscopic preservation, as highlighted within the small fraction of samples showing bone weathering on the surface, but a well-preserved bone tissue microscopically. In addition, the features analysed in our study seem not to be predictive of the depositional context, both when samples with comparable and different PMIs were considered. The only exception seems to be the peculiar pattern identified in the sea and fresh water samples (blackish/darkened areas). However, such a pattern is difficult to interpret and the origin and causes are still unknown. To this purpose, additional investigations are needed by high-resolution technologies, such as backscattered electron imaging in a scanning electron microscope (BSE-SEM), already used in taphonomic studies of bone tissue [10, 15, 21, 23, 51].

Densitometric radiological investigation provided interesting data to taphonomic research. To date, studies focused especially on bone portion “survival” without checking whether differences exist between environments. Therefore, this is the first study in which different depositional contexts are compared and where data on sea water samples are reported.

Differently to anthropological analyses, densitometric radiological investigation highlighted a significant difference between sea water and outdoor samples with old cemeterial cases. The lower values observed in the latter can depend on several variables, among which the environment, age at death, pathology or the bony part we analysed. In fact, the old cemeterial group was the sample mostly represented by tibiae (50% of the sample compared to the 18–27% of the other groups) where the BMD values found were clearly lower than those observed on femurs. Because it was difficult to verify all the variables and especially considering the decrease of the mineral content in the old population already proved in literature [52‐54], we decided to select ten individuals from the same environment (burial in coffin) whose age at death and kind of bone were comparable with the outdoor and sea water samples (age range: 20–43 years old, bone: femur). The obtained BMD values at the middle diaphysis ranged between 1.456 and 1.785 g/cm² (mean: 1.598 g/cm², SD: 0.116 g/cm²), and no differences were observed when comparing these values to those obtained from sea water samples and outdoor cases, excluding any difference among these environments. When fresh water samples were also considered, even if in a small number, similarities were observed with the majority of the samples, highlighting once more the difference with the old group.

Since the sea water samples were all collected from male individuals and considering the well-known differences in bone mineral density existing between males and females [37, 52], comparisons were also carried out limiting the analyses to the only male fraction. Also for this sub-analysis we found similar conclusions, even if cemeterial (young individual), and outdoor samples showed values slightly greater than sea water bones. These variations may be related to the reduced number of samples and/or the position of bodies within the ship. In support to the second hypothesis, BMD values recorded suggested a decrease of bone mineral density in samples collected from individuals recovered in the sea bottom and cargo, while greater values were observed in samples recovered from the deck. However, sample size has to be increased in order to check this variation and to identify the variables involved.

The present study has some limitations. Some samples, especially those buried in coffins, had a PMI longer than the other cases, and no data were available 1 year after death. However, since a well-preserved bone tissue has been macroscopically and microscopically observed in the majority of the cases, we could assume a bone tissue preservation comparable to the other samples even in the early post-mortem period. Concerning bone densitometric investigation, no information was available about BMD at the time of death, preventing to carry out a longitudinal analysis for this parameter assessing its possible changes over the time in each environment. Therefore, we focused mainly on the data after the recovery in the several depositional contexts in order to verify if any difference exists and if BMD can be considered a relevant predictive marker of the depositional environment in forensic cases. An additional limitation lies in the unequal number of samples from each post depositional contexts and especially the small size of the fresh water sample. This requires in the future further analyses on larger sample groups in order to verify the observations reported. Nevertheless, even if no additional data are available, the present study would provide an initial overview of the state of preservation of the bone tissue comparing samples with different post-mortem histories and by adding radiological densitometric methods to the common anthropological investigations used in bone taphonomy research.

The present pilot study represents a first taphonomic attempt to compare samples of contemporary human remains recovered from different environments and PMIs by several perspectives, enriching literature on body and bone preservation, from a macro-, microscopic and radiological point of view. In particular, since no studies are available concerning contemporary human remains recovered from the sea at greater depths such as 400 m and especially in the Mediterranean Sea, these observations, even if on a small sample, provided further data on marine bone taphonomy. The analyses performed highlighted a similar preservation of the bone tissue in the four environments, both macroscopically and microscopically, despite the state of preservation of the body. Similar conclusions were outlined by radiological densitometric investigations, showing no differences among the contexts. Further analyses need to be performed using higher resolution technologies, larger sample sizes, human remains with different PMIs and recovered from various environmental contexts, in order to possibly highlight changes and differences for longer period of times and to obtain a more complete taphonomic picture.

In the meantime, the results obtained in this study suggest that no differences exist among the four contexts at the time of recovery and that neither the bone preservation nor the mineral content/density can be considered predictive of the depositional environment, especially for cases with PMI shorter than 14 months.