Mesolithic Shell Midden Research Articles

Sclerochronological and geochemical study of the carpet shell Ruditapes decussatus in archaeological contexts: A potential tool for season of collection and coastal paleo-temperature

Specimens of the carpet shell Ruditapes decussatus from the Mesolithic shell midden (6th millennium BC) of Beg-an-Dorchenn (Brittany, France) were studied in order to assess their period of collection as well as provide some insight on Paleo-temperature reconstructions. Cross sectioned shells display very clear growth structures allowing an assessment of a growth rhythm of 2 increments per lunar day. From this temporal framework, daily growth increments and winter annual growth breaks were characterized. Our results show the R. decussatus shells were typically collected in the early spring period. Chemical analyses of shell carbonates were also performed to assess a preliminary range of potential sea temperature data from oxygen isotopes values (δ18Oshell). δ18Oshell shows seasonal variations but do not record the whole seasonal temperature range as R. decussatus have a growth break during winter. However, results show predicted SSTs within the range of expected values. Thus, R. decussatus is particularly suited for paleo-environmental reconstructions at high temporal resolution such as seasonal timescales. Additional sclerochronological and chemical research on additional carpet shells and also other mollusk species from Beg-an-Dorchenn or other Mesolithic shell middens along the European Atlantic coast should be considered. Such studies will contribute to the understanding of the seasonal occupation patterns of the last hunter-gatherers of the European Atlantic coast as well as the climate variability at the Mesolithic-Neolithic transition.

Pioneer Archaeologists and the Influence of Their Scientific Relationships on Mesolithic Studies in North Iberia

Abstract Count Vega del Sella’s research marked the beginning of the long tradition of Mesolithic studies in Asturias (North Spain). Another pioneer, L. Sierra, explored a number of caves in Cantabria with Mesolithic shell-middens, but no specific research line was developed on this kind of deposit in the province. The early excavation at Santimamiñe Cave recorded the Basque Country’s first shell-midden, which was thought to belong to a different facies from the deposits in Asturias (Asturian shell-middens). Nevertheless, no line of research into the Mesolithic was developed in that area either. The first research on the Mesolithic in North Iberia did not take place in isolation from European Prehistoric studies at the time. In fact, the historiographical context was characterized by the existence of cordial relationships between the pioneers in this region and several foreign archaeologists. This feedback between Spanish and foreign archaeologists sometimes had a significant impact. In contrast, the relationships between Spanish authors were less intense. This situation and the recurring lack of information on the Mesolithic in Cantabria and the Basque Country did not encourage debate on the existence of different Mesolithic facies in northern Iberia. So for decades the Asturian and the eastern Mesolithic recorded in Cantabria and the Basque Country were regarded as independent objects of study. This indicates the relevance of the scientific relationships and other factors in the perception of the Mesolithic that took shape in the first decades of the twentieth century and which has been perpetuated until the present time.

Why Mesolithic Populations Started Eating Crabs on the European Atlantic Façade Only Over the Past 15 Years?

AbstractMesolithic populations from coastal areas are known as hunter-gatherer-fishers. This way of life is visible in the landscape owing to the presence of large accumulations of shells named shell middens. These anthropogenic refuse heaps are composed of high proportions of marine resources, yet studies dedicated to marine components are relatively recent. Efforts have been made to record marine molluscs, but other minorities are still invisible as a result of the small sizes of archaeological remains. Crustaceans are one of these minorities and this article will focus on one of them: crabs. Why are these decapods persistently ignored by most of archaeologists? Is this due to the scant presence of their remains in Mesolithic shell middens? An overview of published data on crabs is presented here for the Mesolithic period on the European Atlantic façade. The proportions of shell middens comprising these crustaceans is evaluated, as well as quantities and identified species. We endeavour to identify the potential impact of excavation methods and sampling on our knowledge of this marine resource. We present the methods developed to extract data from small archaeological crab remains and show how, from a fragment of a crab finger extremity, we can identify the species and estimate the original size of crabs. This methodological challenge has a major impact on our knowledge of past maritime populations.

Thermo-microstratigraphy of shells reveals invisible fire use and possible cooking in the archaeological record

The archaeological visibility of hearths related to shellfish cooking methods is limited, particularly in pre-ceramic shell midden contexts. Important evidence for use of fire is the thermal alteration of components, namely the identification of burnt shells. Mollusk shells that mineralize as aragonite are particularly indicative of burning due to the conversion of aragonite to calcite through recrystallization at known temperature thresholds. However, roasting temperatures needed to open bivalves, do not necessarily cause thermal alterations in the cooked shell. This complicates the significance of shell mineralogy by itself to recognize cooking, and discerning pre-depositional from in situ heating. To distinguish between cooking and burning, we combine micromorphological analyses with microscopic Fourier transformed infrared spectroscopy to investigate mineralogical thermo-alterations alongside microstratigraphic formation studies. Experimentally heated specimens of Cerastoderma edule and Scrobicularia plana are used to identify the temperature thresholds of biogenic calcium carbonate phase alteration at the micro-scale. These results are then used to interpret mineral alterations in deposits from two Mesolithic shell midden contexts from Portugal. Micro-stratigraphically controlled mineralogy proved to be particularly useful to distinguish between pre-depositional heating from in situ heating, configuring a novel methodology for recognition of traces of cooking shellfish versus traces of fire used for other purposes. Mapping the mineral phase conversion at a micro stratigraphic scale also allows us to identify instances of in situ fire events that were invisible macroscopically. This combined microstratigraphic and mineralogical methodology considerably increases our capacity of deciphering intricate shell midden stratigraphy and occupational events.

A multidisciplinary approach to characterise the Early-Middle Holocene palaeoenvironmental evolution of the Sado Valley of Portugal: Implications for late Mesolithic human communities

In this paper we present the results of a multidisciplinary study performed in the Carrasqueira valley, a tributary of the River Sado (SW Portugal), aimed at characterising the Holocene environmental conditions during the late Mesolithic occupation of this valley. Our findings are based on a 13.5 m long sediment core (Arez3) collected on the alluvial plain close to a late Mesolithic shell midden, the Arapouco site. The results of the multiproxy analyses (texture, magnetic susceptibility, organic composition and chemistry, n-alkanes and palynology) point to a greater marine influence between ca. 8850 cal yrs. B.P. (at the core base) and ca. 7450 cal yrs. B.P. (at 750 cm below mean sea level (MSL)) and the existence of an environment similar to the present-day central estuarine basin. At this point in time, sedimentation rates were lower than the rate of sea-level rise, resulting in the formation of a drowned area with intertidal environments developing on the less incised margins. After 7040 cal yrs. B.P. the contribution of organic matter from terrestrial plants and freshwater phytoplankton to the sediment increased, reflecting a change in the sedimentary pattern, with the estuarine environments progressively giving way to freshwater environments. After the Middle Holocene (ca. 6530 cal yrs. B.P.), negative shifts of δ15N to values ~0‰ point to hyper-eutrophication and cyanobacteria bloom episodes under backswamp conditions. According to these results, the estuarine environment prevailed in the area until 7040 cal yrs. B.P. (5090 cal yrs. B.C.; 390 cm below MSL), i.e., during the Mesolithic occupation of the valley, allowing for the occurrence and for the exploitation of marine shellfish and fish by these hunter-gatherer communities at the proximity of the downstream occupation areas.

Mummification in the Mesolithic: New Approaches to Old Photo Documentation Reveal Previously Unknown Mortuary Practices in the Sado Valley, Portugal

Recently rediscovered photographs of the remains of thirteen individuals buried in the Sado Valley Mesolithic shell middens of Poças de S. Bento and Arapouco, excavated in 1960 and 1962, show the potential of revisiting excavation archives with new methods. The analysis, which applies the principles of archaeothanatology and is enriched by experimental taphonomic research, confirmed details concerning the treatment of the dead body and provided new insights into the use of burial spaces. Some bodies may have been mummified prior to burial, a phenomenon possibly linked to their curation and transport, highlighting the significance of both the body and the burial place in Mesolithic south-western Portugal.

Multidisciplinary investigation reveals an individual of West African origin buried in a Portuguese Mesolithic shell midden four centuries ago

Cabeço da Amoreira is a well-studied shell midden with a robust chronology based on a large number of radiocarbon dates on Mesolithic human burials. Surprisingly, we discovered one individual that lived about 400 years ago buried in this site. We employed a multidisciplinary approach integrating archaeology, historical records, genetics, radiocarbon dating and stable isotope analysis to investigate the biogeographic origins of this individual and burial circumstances. We could determine that this was a man of West African origin, probably from Senegambia, arriving in Portugal via the Trans-Atlantic Slave Trade. Our study provides new insights into aspects of the life and death of a first-generation African individual in Portugal during the Trans-Atlantic Slave Trade period and highlights the power of multidisciplinary research to unravel unwritten history.

Use of Different Digitization Methods for the Analysis of Cut Marks on the Oldest Bone Found in Brittany (France)

Archaeological 3D digitization of skeletal elements is an essential aspect of the discipline. Objectives are various: archiving of data (especially before destructive sampling for biomolecular studies for example), study or for pedagogical purposes to allow their manipulation. As techniques are rapidly evolving, the question that arises is the use of appropriate methods to answer the different questions and guarantee sufficient quality of information. The combined use of different 3D technologies for the study of a single Mesolithic bone fragment from Brittany (France) is here an opportunity to compare different 3D digitization methods. This oldest human bone of Brittany, a clavicle constituted of two pieces, was dug up from the mesolithic shell midden of Beg-er-Vil in Quiberon and dated from ca. 8200 to 8000 years BP. They are bound to post-mortem processing, realized on fresh bone in order to remove the integuments, which it is necessary to better qualify. The clavicle was studied through a process that combines advanced 3D image acquisition, 3D processing, and 3D printing with the goal to provide relevant support for the experts involved in the work. The bones were first studied with a metallographic microscopy, scanned with a CT scan, and digitized with photogrammetry in order to get a high quality textured model. The CT scan appeared to be insufficient for a detailed analysis; the study was thus completed with a µ-CT providing a very accurate 3D model of the bone. Several 3D-printed copies of the collarbone were produced in order to support knowledge sharing between the experts involved in the study. The 3D models generated from µCT and photogrammetry were combined to provide an accurate and detailed 3D model. This model was used to study desquamation and the different cut marks, including their angle of attack. These cut marks were also studied with traditional binoculars and digital microscopy. This last technique allowed characterizing their type, revealing a probable meat cutting process with a flint tool. This work of crossed analyses allows us to document a fundamental patrimonial piece, and to ensure its preservation. Copies are also available for the regional museums.

Testing the reliability of CT scan-based dental wear magnitude scoring.

Digital models are now frequently used in biological anthropology (bioanthropology) research. Despite several studies validating this type of research, none has examined if the assessment of dental wear magnitude based on Computerized Tomography (CT) scans is reliable. Thus, this study aims to fill this gap and assess if dental wear magnitude scoring based on CT scans provides results consistent with scoring based on direct observation of the physical specimens. Dental wear magnitude from 412 teeth of 35 mandibles originating from the Portuguese Muge and Sado Mesolithic shell-middens was scored. The mandibles were also CT scanned and visualized using 3D Slicer. CT scan-based scoring of dental wear magnitude was then undertaken. Two scoring rounds were undertaken for each observation method (totaling four scoring rounds) and an intra-observer error test was performed. The averaged results of the two observation methods were compared via boxplots with paired cases. Intra-observer error was negligible and non-significant. Scoring results are comparable between the two observation methods. Notwithstanding, some differences were found, in which CT scan assessment generally overestimates dental wear when compared to direct observation. Our results generally validate the use of CT scans in studies of dental wear magnitude. Notwithstanding several caveats relating to CT scanning and visualization limitations should be considered to avoid over or under-estimation of dental wear.

Late Atlantic Settlement in Southern Portugal - Results of an excavation of a Mesolithic shell midden by the River Sado

The study of large settlement sites with graves from the Late Mesolithic has changed our conception of this period. In western Europe this kind of antiquity has long been known, and it is well represented in the coastal area of western Iberia. One settlement site —Popas de Sao Bento, near the River Sado in southern Portugal — has recently been excavated as part of a joint Swedish-Portuguese project. The results of the excavation give interesting perspectives on specific and general conditions in a broader geographical, chronological, and social context.

Submerged landscapes, marine transgression and underwater shell middens: Comparative analysis of site formation and taphonomy in Europe and North America

Shell middens, sometimes in the form of mounds of great size, are a ubiquitous indicator of coastal settlement and exploitation of marine resources across the world. However, shell middens are relatively rare before the mid-Holocene because most palaeoshorelines before that time are now submerged by sea-level rise since the Last Glacial Maximum (LGM). Previously reported examples of underwater shell middens are almost unknown and of uncertain status, and it has generally been assumed that such deposits would not survive the destructive impact of sea-level rise or would be indistinguishable from natural shell deposits. Recently, two examples of underwater shell deposits have been independently discovered and verified as anthropogenic midden deposits – a Mesolithic shell midden on the island of Hjarnø in the Straits of Denmark, and a Middle to Late Archaic shell midden in the Econfina Channel of the Gulf of Mexico, Florida, USA. We report the comparative geoarchaeological analysis of these deposits, using a sedimentological approach to unravel their formation history and post-depositional transformation. Despite the differences in coastal geomorphology and geology, cultural context, molluscan taxonomy and preservation conditions between these sites, the results demonstrate similar sedimentological profiles that are distinctive of anthropogenic deposits, demonstrate their origin as subaerial deposits at the shore edge before inundation by sea-level rise, and show that these properties can be identified in sediment samples recovered from coring. These findings support arguments that such sites likely exist in greater numbers than previously assumed, that they can be identified from minimally invasive techniques without the need for extensive underwater excavation, and that they should be sought to fill critical gaps in the temporal and geographical record concerning Late Quaternary human use of coastal zones and marine resources.

A drowned Mesolithic shell midden complex at Hjarnø Vesterhoved, Denmark and its wider significance

Anthropogenic shell accumulations (shell middens), often of great size, occur in their tens of thousands around the world’s coastlines. They mostly date from the Mid-Holocene onwards and are frequently taken as symptomatic of a Postglacial ‘revolution’ involving world-wide population growth and intensification in exploitation of marine resources. However, the comparative rarity of earlier deposits may have as much to do with Postglacial sea-level rise and the loss of evidence from earlier palaeoshorelines as with genuine socio-economic trends. Here we investigate the underwater Mesolithic (Ertebølle) shell midden of Hjarnø Vesterhoved in Denmark, one of the first underwater shell middens to be systematically verified as an anthropogenic shell deposit in a region world-famous for its many hundreds of Ertebølle shell mounds on the present shoreline. We show how a combination of geophysical survey, coring, excavation, stratigraphic interpretation and macroscopic analysis of midden contents can be used to identify underwater deposits, to unravel their taphonomic and post-depositional history in relation to surrounding sediments, and to distinguish between cultural and natural agencies of shell accumulation and deformation. We demonstrate the presence of an intact underwater shell-midden deposit dated at 5400–5100 cal BC, one of the earliest in Denmark. We demonstrate the usefulness of such material in giving new information about early coastal subsistence economies and greater precision to the measurement of palaeo-sea levels. We discuss the implications of our results for an improved understanding of the Mesolithic record in Denmark and of biases in the archaeological record of Late Pleistocene and Early-to-Mid Holocene coastal contexts. We emphasise the importance of researching more fully the geomorphological and taphonomic processes that affect the accumulation, destruction, burial, preservation and visibility of underwater archaeological deposits, the need to extend underwater investigations more widely and to more deeply submerged palaeoshorelines, and the combination of methods required to advance such investigations.

New paradigms in the exploitation of Mesolithic shell middens in Atlantic France: The example of Beg-er-Vil, Brittany

The Atlantic coast of north-west France is one of the classic shell-midden regions of the European Mesolithic, made famous by the excavations of Téviec and Hoedic in the first half of the 20th century. At this time, there was a lack of interest in the food refuse component of shell middens. By the end of the 1990s new study methods and techniques had also contributed to a better description of the varied activities of these coastal populations. In Atlantic France, new excavations have demonstrated that shell middens are not a site type but rather one of a variety of stratigraphic units that make up the total settlement pattern. Our perception of the Mesolithic hunter-gatherers of the French Atlantic coast has now changed from a population pre-occupied with day-to-day survival and forced to eat shellfish out of necessity, to fisher-hunter-gatherers involved in varied activities. Their knowledge of marine biotopes is revealed by the diversity of marine animals dedicated to food, but also by the collection of other raw materials washed up on the beach , including flint or shells devoid of flesh. The latter give us access to the symbolic sphere and were clearly and carefully selected for ornamental purposes.

O mesolítico em Portugal: uma nova visibilidade para os concheiros do rio Sado

Com base na investigação realizada para o projecto de Seminário de Licenciatura em Arqueologia da Universidade de Lisboa, vêm-se aqui apresentar e aprofundar parte dos resultados obtidos para a melhor compreensão da realidade mesolítica da bacia do Sado. Assim, com recurso aos SIG, realizou-se um estudo incidente nos concheiros mesolíticos do rio Sado, nos quais se estudou a relação das implantações dos sítios com o meio geográfico, procurando observar características comuns de forma a encontrar a possível existência de um padrão de assentamento. Optou-se por destacar neste trabalho a ligação com o espaço hidrográfico, raiz comum das implantações dos concheiros do Sado, assim como os aspectos relativos à visibilidade entre os próprios sítios, observando-se a fraca intervisibilidade entre os vários grupos ao invés do inicialmente ponderado.

Annual Growth Patterns and Interspecimen Variability in Mg/Ca Records of Archaeological Ostrea edulis (European Oyster) from the Late Mesolithic Site of Conors Island

Annual growth patterns in marine mollusc shells are valuable indicators of the condition of marine ecology through time. In archaeological contexts, the mollusc’s time of death (i.e. the last season of growth) is an indicator of human exploitation patterns throughout the year, enabling the reconstruction of when and how often gathering occurred as well as when sites were occupied. Both pieces of information, growth rate and season of death, are vital for understanding exploitation pressure(s) in the past, and building baselines for modern environmental policies that secure sustainable marine resources. Previously, these parameters have been determined by incremental growth-line or isotopic analyses, which are time consuming and often expensive techniques, thus restricting sample size and the overall robustness of palaeoecological interpretations. Here, we apply Laser Induced Breakdown Spectroscopy (LIBS) to produce elemental maps (Mg/Ca) with the potential to trace and display growth patterns quickly, and at a reduced cost. We further compare the elemental maps with the results obtained from incremental growth-line analysis to provide a structural context for the geochemical data, and demonstrate the utility of an integrated methodological approach. Our pilot study was undertaken on 12 European oysters (Ostrea edulis, Linnaeus, 1758) from the Late Mesolithic shell midden at Conors Island, Co. Sligo in the Republic of Ireland. Our LIBS analysis enabled us to accurately and quickly determine repeating growth patterns, which were often in agreement with the annual growth increments visible through the microscopic analysis. Based on this comparative dataset, including structural and geochemical patterns, the Late Mesolithic site of Conors Island had been occupied throughout the year. Moreover, our analyses highlight the applicability of LIBS to determine prehistoric seasonality practices as well as biological age and growth at an improved rate and reduced cost than was previously achievable.

At the water's edge: Micromorphological and quantitative mineral analysis of a submerged Mesolithic shell midden at Hjarnø Sund, Denmark

This study presents the first application of micromorphology and Automated Scanning Electron Microscopy (ASEM) to a submerged shell-bearing midden site in Denmark, one of the few submerged shell-matrix sites in the world to have undergone archaeological excavation. The use of micromorphology in these deposits provides a means to distinguish primary and secondary deposits and degree of preservation. The additional input of quantitative mineral analysis provides an empirical tool for correlation of minerogenic components across the site and assessing change through time. Micromorphological insights into the formation history and post-depositional modification include epizootic infestations within oyster shells at the base of the profile and needle-fibre calcite at the top - the former is indicative of accumulation of at least part of the shell midden below water, whilst the latter is indicative of exposure. Aside from the shell itself, other cultural traces include heated and worked flint fragments, burnt and unburnt hard bone fragments (including fish bone), soft (spongy) bone, charcoal (Ulmus sp., Alnus sp., Corylus sp.), and preservation of what could possibly be antler velvet. Quantitative mineral analyses provide support for the successive stabilisation of the midden site by accumulating minerogenic (silica-rich) sands over the glacial clay and detrital gyttja deposits (defined by smaller grain size, greater heavy mineral content), and subsequently between the shell matrix itself, until the erosion of these protective sediments in more recent times. Further micromorphological profiles are needed to fully assess local versus regional signatures within the site, which otherwise provides an ideal context against which to compare the effects of inundation in similar microtidal contexts.

From Oysters to Cockles at Hjarnø Sund: Environmental and Subsistence Changes at a Danish Mesolithic Site

ABSTRACTArchaeological fieldwork at Hjarnø Sund in Horsens Fjord (eastern Jutland, Denmark) has explored an eroding Mesolithic shell midden. Its stratigraphy is characterized by two layers, containing marine mollusk taxa typically collected by Mesolithic hunter-gatherers for food. In the field, the lower layer appeared to be dominated by oysters (Ostrea edulis), while the upper one by cockles (Cerastoderma edule), which was confirmed by our zooarchaeological study. Accelerator mass spectrometry radiocarbon (AMS 14C) dating on shells and paired charcoal samples from the two layers indicate that these are chronologically consecutive (separated by as little as 0–163 yr [95.4%]) and that the oyster-to-cockle shift dated between ~5500–5300 and ~5300–5200 cal BC (around or just after the Kongemose/Ertebølle transition). The shell midden at Hjarnø Sund is, thus, one of the oldest-known in Denmark, demonstrating that intensive shellfish exploitation was a hallmark of the Ertebølle culture from its inception. Oyster-to-cockle shifts, thus, also occurred at times other than the Mesolithic–Neolithic Transition and may have been ultimately caused by local shoreline displacements, resulting from changes in sedimentation, possibly induced by drops in relative sea level.

Mesolithic human occupation and seasonality: sclerochronology, δ18O isotope geochemistry, and diagenesis verification by Raman and LA-ICP-MS analysis of Argyrosomus regius (meagre) sagittae otoliths from layer 1 of Cabeço da Amoreira Mesolithic shell midden (Muge, Portugal)

We present preliminary sclerochronological analysis on 15 Argyrosomus regius (meagre) otoliths collected from two different human occupation levels from the Cabeco da Amoreira shell midden (Muge valley, Portugal). The otoliths were sectioned and observed under a reflected light stereomicroscope to examine seasonal growth rings. Carbonates collected from individual growth rings subsampled with a micromill sampling device were analyzed for their stable oxygen isotope content with the objective of determining the predominant season of capture and therefore season of site use, environmental conditions, and sites of resource procurement (local versus regional). The otoliths’ stable isotope records show clear seasonality and a season of capture estimate that seems consistent with a “good season” (warmer season, i.e., from spring to late summer/beginning of autumn in this area) site use in the last occupation layer (the great majority of otoliths are from layer 1), except for four samples giving heaver oxygen isotope (δ18O) values that suggest colder conditions and only one with markedly positive δ18O values. Potential effect of diagenesis on the otolith records was also assessed through coupled laser ablation–inductively coupled plasma mass spectrometry (LA-ICP-MS) and by micro-X-Ray diffraction (μ-XRD). Results revealed the presence of only aragonite and no traces of calcite, providing no evidence of diagenesis that could significantly alter isotope results and lead to erroneous interpretations. The implications of these results are discussed and compared with data from other archeological sites, as well as data from micromorphology regarding continuity or interruption of site use and other faunal remains.

Detecting single events in large shell mounds: A GIS approach to Cabeço da Amoreira, Muge, Central Portugal

During the 2014 archaeological campaign in the Mesolithic shell midden of Cabeço da Amoreira (Muge, Central Portugal) a concentration over a small area, approximately 1m2, of faunal remains with a single taxon (red deer), lithics, charcoal and fire-cracked rocks was identified in one of the shell-rich layers of the mound.Here we present the results of the analysis of bone and lithic materials coming from that feature, as well as refitting data from both classes and correspondent GIS-based spatial analysis, using Kernel Density Estimation and Ripley's K function. All these procedures allowed the identification of non-random density patterns, the definition of clear spatial limits for the feature, and to attest the unique character of the context as a single depositional event within the Cabeço da Amoreira shell mound. Alternative hypothesis for the natural and cultural processes involved in the formation of the feature are suggested, including the possibility that it corresponds to a refuse disposal of a short episode of carcass processing but also that some social-based purposes might have been involved.

The microstratigraphic record of human activities and formation processes at the Mesolithic shell midden of Poças de São Bento (Sado Valley, Portugal)

Shell midden formation is largely controlled by anthropogenic processes, resulting from human exploitation of aquatic resources. This makes shell middens archives of both human behaviour and palaeoenvironmental records. However, their often complex stratigraphy hampers the isolation of individual anthropogenic events. In the central/southern coast of Portugal, extensive inland estuaries were preferential settings for Mesolithic groups from c. 6200 cal BC. Here, we present a microstratigraphic approach to the shell midden of Pocas de Sao Bento, one of the largest and best-known sites in the Sado Valley. The microfacies approach was based on sedimentary components, their abundance and arrangement, and post-depositional processes. Anthropogenic processes identified as tossing events and anthropogenically reworked deposits allowed inferences on spatial organisation, preferential refuse areas, occupational surfaces, and temporality of the occupations. The presence of calcareous pebbles in the anthropogenic, shell-rich sediments, together with foraminifera, presumably from the estuarine marshes, is compared with the regional geology, providing a hypothetical location of the shellfish gathering. The microstratigraphy described reveals a full internal dynamic in the formation of the apparently homogeneous shell midden layer. The human activities inferred at Pocas de Sao Bento have many similarities with those reported for Cabeco da Amoreira in the nearby Tagus palaeo-estuary. This evidence points to the need for further micromorphological approaches in similar deposits. The study of shell midden formation processes, through integrative microcontextual approaches, plays a major role in understanding Mesolithic societies in the large early Holocene estuary environments of Atlantic Iberia.