Abstract
Land subsidence has played and is still playing a significant role in coastal wetlands worldwide and in palaeogeographical reconstructions of such wetlands. The varying thickness of compaction-prone sediments over a stable subsurface is a key factor in determining its magnitude and in locating the most affected areas. In the coastal low-lying Agro Pontino (Lazio, Italy), subsidence of the past 90 years has been mapped using historical elevation data. Due to the fortunate preservation of distal Avellino tephra (AV-tephra, ca. 1900 cal. BCE) within its marshy strata, discovered a decade ago, detailed palaeogeographical reconstruction of the landscape in preparation for an assessment of its land use suitability in the Early Bronze Age (EBA) was possible. Current altitude variations of water-lain tephra in lake areas assumed to be connected necessitated a closer look at its original deposition altitude and the role of post-depositional subsidence. Recent subsidence patterns proved very useful for distinguishing stable from subsidence-prone areas. Two different EBA palaeo lake environments are distinguished: an inland and a near-coastal lake. The AV-tephra altitude variation within these lakes partly marks differential post-depositional subsidence within these lakes. Calculation of initial ripening of tephra-bearing lake deposits on top of shallowly buried Pleistocene ridges allowed for an estimation of original tephra deposition altitudes and associated lake levels. For the inland lake, a wide lake edge zone between 0.5 and 2 m above current sea level (m asl) was reconstructed, where EBA habitation or land use was possible. At the near-coastal lake, a water level of −1.5 to −1.3 m asl at the time of AV-tephra deposition was constrained. Because tephra deposition occurred here just after marine influence ceased, this altitude range is proposed to be a Relative Sea Level (RSL) index point at the time of AV-tephra deposition. The altitude range is in agreement with RSL models for tectonically stable areas in this region. The importance of subsidence in palaeogeographical, water level and RSL reconstructions in the region is stressed.
Highlights
Lowland coastal areas around the world often contain valuable depositional records of their Holocene evolution, which is determined by the interplay between Relative Sea Level (RSL) rise, climate change, alluvial processes and human activities
In the Agro Pontino, Serva and Brunamonte (2007) identified the impact of subsidence by shrinkage and compaction. Our work confirms this subsidence for the inland part of the Agro Pontino (Van Gorp and Sevink, 2019) and demonstrates how its magnitude depends on the depth of the Pleistocene subsurface and lithology of the Holocene sediments
The observed altitude variation of the AV-tephra marker bed in the Agro Pontino is caused by three palaeogeographical factors: (1) the initial gradient of the fluvio-deltaic and lacustrine systems, (2) the initial lake bottom depth of lacustrine and lagoonal units, and (3) post depositional subsidence, both in historic and in recent times
Summary
Lowland coastal areas around the world often contain valuable depositional records of their Holocene evolution, which is determined by the interplay between Relative Sea Level (RSL) rise, climate change, alluvial processes and human activities. Teatini et al, 2011; Fontana et al, 2017) and smaller deltaic and coastal back-barrier regions, such as in Southern England (Long et al, 2006; Massey et al, 2006) and several coastal basins along the Italian coast Examples of the latter are the Arno plain Ferranti et al, 2011), and the Agro Pontino in Lazio (Serva and Brunamonte, 2007; van Gorp and Sevink, 2019), the last of which is the study area of this paper In most of these studies, the palaeogeographical reconstruction of coastal lagoons or back barrier areas with their coastal hinterland involves chronological controls in the form of radiocarbon dating of basal peats, which are the peats that form directly on top of the pre-Holocene marine transgressional surface with rising groundwater level. Intensified reclamation of low-lying coastal areas in historical times but especially in the last century has caused increased subsidence rates in many areas around the world (e.g. van Asselen et al, 2018) and in many instances strongly modified the topography of the pre-reclamation land surface
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