The Decade of Ocean Science for Sustainable Development 2021–2030 is a UN initiative that promotes a common framework for supporting stakeholders in studying and assessing the health of the world’s oceans. The initiative also presents a vital opportunity to improve the integration of archaeology within the marine sciences. With the First Global Planning Meeting of the Decade held in Copenhagen at the National Museum of Denmark in May 2019, steps are already being taken to make the best of this opportunity, and the resulting creation of an Ocean Decade Heritage Network is proposed as a way forward to continue to raise awareness in the cultural heritage community about the Decade and to facilitate information sharing regarding this endeavour.

The Intergovernmental Panel on Climate Change has documented wide-ranging changes to the world's coasts and oceans, with significant further change predicted. Impacts on coastal and underwater heritage sites, however, remain relatively poorly understood. The authors draw on 30 years of research into coastal and underwater archaeological sites to highlight some of the interrelated processes of deterioration and damage. Emphasising the need for closer collaboration between, on one hand, archaeologists and cultural resource managers and, on the other, climate and marine scientists, this article also discusses research from other disciplines that informs understanding of the complexity of the interaction of natural and anthropogenic processes and their impacts on cultural heritage.

Seagrass meadows form valuable ecosystems, but are considered to have low cultural value due to limited research efforts in this field. We provide evidence that seagrass deposits play a hitherto unrealized central role in preserving valuable submerged archaeological and historical heritage across the world, while also providing an historical archive of human cultural development over time. We highlight three case studies showing the significance of seagrass in protecting underwater cultural heritage in Denmark, the Mediterranean and Australia. Moreover, we present an overview of additional evidence compiled from the literature. We emphasize that this important role of seagrasses is linked to their capacity to form thick sedimentary deposits, accumulating over time, thereby covering and sealing submerged archaeological heritage. Seagrass conservation and restoration are key to protecting this buried heritage while also supporting the role of seagrass deposits as carbon sinks as well as the many other important ecosystem functions of seagrasses.

Seagrasses have been entwined with human culture for millennia, constituting a natural resource that has supported humanity throughout this history. Understanding the societal value of seagrass fosters appreciation of these ecosystems, encouraging conservation and restoration actions to counteract historic and predicted losses. This study overviews the plethora of seagrass use in human history, ranging from spiritual and ceremonial roles, direct and indirect food resources, medicines and raw materials, dating back more than 180 000 years. While many past uses have been abandoned in modern societies, others have persisted or are being rediscovered, and new applications are emerging. As these uses of seagrasses depend on harvesting, we also underscore the need for sustainable practices to (re)generate positive interactions between seagrasses and society. Our review contributes to revalue seagrass societal ecosystem services, highlighting ancient and more recent human and seagrass relationships to incentivize conservation and restoration actions.

The preservation potential of sediments from a submerged prehistoric site buried in a full marine environment was assessed using a combination of direct in situ measurements, measurements on extracted sediment cores and laboratory mesocosm experiments. The results show that first and foremost it is paramount to ascertain the state of preservation of the materials in the seabed—in this case wooden artifacts—that are to be preserved in situ. The results suggest that dissolved oxygen and sulfide are good parameters to measure in situ to assess the general oxidizing or reducing nature of the environment. If it is possible to take sediment core samples, it is similarly important to measure dissolved oxygen and hydrogen sulfide in the core and extract pore water to assess for sulfate content. Sediments should be characterized for particle size, water content (porosity) and organic content. In this way, we show that dissolved oxygen was rapidly depleted in the first few centimeters of sediment, thereafter sulfate reduction dominated the geochemical processes. Coarser sediment types with high porosity and low organic content have lower rates of organic material turnover, indicating better preservation conditions.

Using the Mesolithic site of Tudse Hage in the Great Belt of Denmark, this paper proposes a generic stepwise process to create geoarchaeological models that output seamless morphology maps in a GIS. This was achieved using remote sensing databases and the collection of marine geophysical data, above and below the seabed. On the basis of these data, key areas, with sediment sequences representative of the postglacial transgression surfaces, were identified. Core samples were taken for palaeoenvironmental analysis and dating that enabled a reconstruction of the relative sea-level changes. Using this information, palaeogeographic coastline maps of the Kongemose, late Kongemose, Ertebølle, and Neolithic periods in the Tudse Hage area were prepared, and potential hotspots for archaeological sites were proposed. Since their inundation, submerged prehistoric archaeological sites have been, and are, dynamic, with anthropogenic and natural processes affecting their stability and preservation. With the advocation of in situ preservation as a means of managing underwater cultural heritage, predicting where sites have survived these processes, and where they can be found, in advance of subsea development or other anthropogenic exploitation, is essential. Future natural threats to sites preserved in situ were determined through the modelling of seabed currents and sediment erosion.