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Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2022; Aachen : RWTH Aachen University 1 Online-Ressource : Illustrationen, Diagramme, Karten (2023). = Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2022, In times of global climate change and subsequent eustatic sea-level rise, coastal communities are in need of reliable models of future sea-level change. Since instrumental observation of sea level only spans the most recent centuries, other archives are necessary to identify long term trends and extrapolate these into the future. Anything that forms with an undeniable spatial relationship to mean sea level and is preserved over geological times can function as an archive of paleo sea-level. Examples are biological, like corals, geomorphological, like coastal notches or archaeological, like ancient harbours. Another one of these archives, or so-called sea-level indicators, is beachrock.Beachrocks are beach sediments that lithified through a natural cementation with carbonate (or rarely silicate) minerals. The cementation of beachrock can take place rapidly, on a scale of years rather than millennia, and is most commonly observed on tropic and subtropic coastlines. When a beachrock is removed from its coastal context, either elevated significantly above mean sea level or submerged below, its textures, cements and age can yield information on the paleo sea-level of the coastline in question.When reconstructing paleo sea-level from a sea-level indicator, two attributes are of particular interest: the age of the indicator and the elevation change it underwent since formation. Both of these are problematic when the indicator is beachrock. While dating of beachrock is a problem that may or may not be solved by advancing dating technologies, the elevation change is at the centre of this work. To quantify the elevation change that beachrock underwent post formation, its initial relationship with mean sea-level needs to be known.Sedimentological facies analysis is a way to interpret the initial position of beachrock relative to mean sea level by studying its textures, structures, fossils content, cements and other characteristics. Although its benefits are generally recognised, beachrock facies often remain under-described in the literature. In this thesis, beachrocks from two coastlines, northeastern Oman and northwestern South Africa were investigated for their suitability as sea-level indicators.In Oman, one study area shows a set of twelve marine terraces that reach a height of 500 m above mean sea level. The upper terraces are erosional and beachrocks are the only sediment-cover that withstood erosion. The lower terraces, on the other hand, are depositional and beachrocks occur alongside other sediments, which can be fluvial, alluvial, or marine. Sedimentological macro- and microfacies analysis was carried out on five outcrops on three terrace levels and three outcrops south of the terraced coastal section. This work led to the identification of eight lithofacies in six facies associations. It was demonstrated that facies analysis can lower the indicative meaning of beachrock significantly. Former models of beachrock interpretation were completed by ichnological information and observations within gravel dominated beachrocks. The data collected on beachrocks south of the terraced coastline in Sur, led to a survey of the stratigraphical relationship of beachrocks with coastal notches, bedrock fanglomerates, and biological indicators of paleo sea-level. The oldest paleo notches in Sur stem from the last interglacial sea-level highstand of MIS 5e. This is concluded from cosmogenic nuclide dating of the fanglomerate bedrock in Sur Lagoon. All outcrops of paleo notches around Sur Lagoon were investigated in regards to the faunal distribution and notch shape. Furthermore, the notches’ absolute elevation and biological markers relative to msl were measured with a differential GPS. The bioerosional notch occurs at the same height around Sur Lagoon, indicating that the area remained tectonically stable over the last 125 kyr. According to the elevation of the notch-apex, msl was 3.93 ± 0.12 m higher than today during the last interglacial. The distribution of boring and bioconstructing organisms relative to the notch shape displays at least one phase of short-term sea level rise subsequent to the notch formation. The beachrocks that are associated with the bioerosion notches in Sur Lagoon show a larger grainsize than any sediment that is deposited in the lagoon nowadays. This, in combination with the occurrence of exceptionally high and deep abrasion notches, indicates that the coastline was more openly exposed and thus experienced a higher wave energy during the notch formation.At Mission Rocks on the KwaZulu-Natal coastline of South Africa, a narrow beach with isolated sand patches occupies low points of an otherwise continuous 3 m thick, raised shore platform of sandy and gravelly beachrocks. These beachrocks are in the process of disintegrating into megagravel deposits through chemical and mechanical weathering in a wave-dominated, high-energy setting. The breakdown is potentially slowed by a contemporary, fast-forming beachrock facies that blankets the surface and fills fractures and potholes within the older platform. The accumulation and cementation of this recent beachrock was investigated as part of this thesis. The beachrock is dated by historical evidence to post-World War II. Data from field observations, petrographic, and geochemical methods reveal that the cementing agents of the beachrock were precipitated from marine water in a phreatic setting despite its position above the intertidal zone. Not only does this facies have implications for the interpretation of paleo beachrock as a sea-level indicator, it also raises further questions regarding modelling of coastal erosion of beaches associated with outcrops of beachrock., Published by RWTH Aachen University, Aachen

We investigated the potential of mangrove pollen from Mahe, Seychelles, to improve existing metre‐scale Late Holocene sediment‐based sea‐level reconstructions. Mangrove species at two mangrove sites are broadly zoned according to elevation within the tidal frame. Modern pollen rain from traps deployed for 1 year generally have a poor relationship with modern vegetation, and relatively low pollen production rates. Pollen from mangrove species that live in narrow elevation zones (e.g. Avicennia marina) are poorly represented in modern pollen rain, while pollen from mangrove species that live across a larger elevational range (e.g. Rhizophora mucronata) are relatively well represented. Pollen was found in extremely low concentrations in mangrove surface and core sediments, which inhibited further study into pollen transport and preservation. The results from this modern study demonstrate that utilizing mangrove pollen would not decrease existing metre‐scale vertical uncertainties in Late Holocene sea‐level reconstructions in the Seychelles. We suggest that this approach may still be successful in other locations if mangrove vegetation is (i) zoned at a more extensive lateral scale and (ii) is closely associated with modern pollen rain and surface sediments, and (iii) sedimentological conditions promote the preservation of pollen in fossil sequences.

Beachrocks are coastal sediments that are lithified through the precipitation of carbonate cements. It is widely acknowledged that lithofacies in beachrocks are variable and their interpretation is useful when using beachrock as a sea-level indicator. Surprisingly, the facies variability of beachrocks remains understudied as they are almost exclusively described as seaward dipping, sandy, slab-shaped outcrop forming in low energy dissipative beach environments. The Mission Rocks coastline of South Africa is in stark contrast. Here the coast comprises an up to 3 m thick platform of beachrock, where a variety of sedimentological facies are observed. The northeastern coastline of Oman is rich in unusually coarse beachrock conglomerates and also beachrocks showing a diverse ichnological community. In our studies we aim to use sedimentological facies analysis, petrography and SEM to unravel the deposition- and cementation processes of several beachrock facies. Beachrock lithofacies that can be linked to a certain sea-level are then utilised to refine the current model of beachrock facies interpretation used in sea-level research. However, some of the newly described facies raise questions about the accuracy of beachrock facies interpretation. In particular, an unusual beachrock breccia interposed amongst the breakdown remnants of the platform at Mission Rocks beach comes to mind. The breccia documents a cycle of simultaneous erosional breakdown and depositional build-up of the beachrock platform, a yet undescribed process for the development of beachrock, and inhibits a cement mineralogy indicative of marine phreatic cementation while forming in the supratidal zone.

The global population living within low lying coastal zones face significant challenges in the coming century as sea levels rise. Unfortunately, the acceleration and magnitude of the rise is still a subject of debate, leaving coastal communities often paralyzed in the decision whether to mitigate or abandon. In order to better elucidate future sea-level rise, process analogues from the geological record are used to constrain model predictions. The Last Interglacial (LIG, ca. 128–116 ka), represents the most recent period when Earth’s climate was warmer and sea levels were higher than today. Such insights are derived from the global accumulation of geological sea-level proxies, called paleo relative sea level (PRSL) indicators that are comprised of accurate chronological and elevation constraints as well as considerations for post-depositional processes. However, the spatial and temporal coverage of PRSL indicators is uneven, especially within Eastern Africa and the Western Indian Ocean (EAWIO). In order to establish the current understanding of LIG sea-level in the EAWIO, this dissertation compiles a sea-level database using published LIG geological sea-level proxies. The EAWIO comprises of extensive tropical coastlines and coralline islands with many occurrences of well preserved LIG sequences. These LIG sequences vary in form from islands entirely composed of Pleistocene reefs (i.e., Aldabra, the Seychelles) to uplifted marine terraces in the north of Somalia to Marine Isotope Stage (MIS) 5 tidal notches in Mozambique. This effort uses theWALIS interface and contains 58 sea-level indicators and 2 terrestrial limiting data points. Studies within this database stretch from the 1960s until present, with varying degrees of PRSL precision. Taking this into account, this dissertation explores the advantages of applying modern chronological constraints and advanced surveying techniques to a classic Pleistocene site along the southwestern shores of Madagascar. Here, new investigative techniques provide an updated late Pleistocene geomorphological understanding of southwestern Madagascar. Through the use of Structure from Motion / Multi-View Stereo in combination with differential Global Navigation Satellite System surveys, 3D outcrop reconstructions of sequential geomorphologcial zones and their intrinsic facies are presented. This sequence is chronologically constrained using U-series ages from both in situ and reworked coral samples. From this chronologically sequence, the LIG in Lembetabe was initiated by the deposition of a fringing reef that reaches approximately 2 m above mean sea level. This fringing reef environment persisted throughout the LIG, before a combination of slow regression and changes in the sedimentation regime covered the fringing reef with intertidal and beach sediment. A final close-out of the system occurred as sea level fell further and dune fields began to migrate offshore before. This interpretation of the Lembetabe LIG sequence is then examined through the use of Forward Stratigraphic Models (FSMs). Currently, FSMs are commonly deployed within large-scale basinwide characterizations for industry. By scaling-down the FSM DIONISOS, a suite of synthetic Quaternary subtropical fringing reefs in southwestern Madagascar are produced. Through this approach, each reef is subjected to a distinct Greenland Ice-Sheet (GrIS) and Antarctica Ice-Sheet (AIS) melt scenarios produced by a coupled ANICE-SELEN glacial isostatic adjustment (GIA) model. These scenarios match LIG sea-level curves previously hypothesized within the Indo-Pacific Basin. The resulting collection of synthetic reef sequences so dramatic shifts in both sedimentation and preservation regimes along the Lembetabe coast. By establishing a broad compilation of PRSL indicators at a basin scale, the reexamination of a previously described PRSL indicator, and finally the evaluation of outcrop interpretation, this dissertation provides a new PRSL benchmark for the EAWIO and a new workflow for the evaluation of PRSL indicators along carbonate coasts.

An understanding of the modern relationship between diatom species and elevation is a prerequisite for using fossil diatoms to reconstruct relative sea level (RSL). We described modern diatom distributions from seven transects covering unvegetated subtidal environments to forested uplands from four tidal wetland sites (Smith Creek, Bone River, Niawiakum River, and Naselle River) of Willapa Bay, Washington, USA. We compared our diatom dataset (320 species from 104 samples) to a series of environmental variables (elevation, grain-size, total organic carbon (TOCSOM), and porewater salinity) using hierarchical clustering and ordination. While no single variable consistently explains variations in diatom assemblages at every site, elevation, salinity, and substrate (mud fraction and TOCSOM) are variables affecting diatom distribution along our transects. Elevation was the major environmental control of diatom variability (explained 27–39% variance) at four transects (Bone River Transect 1, Niawiakum River Transect 2, Naselle River Transect 1 and 2). Salinity and substrate were the major environmental controls (explained 12–32% variance) of diatom variability at three transects (Niawiakum River Transect 1: salinity; Smith Creek Transect and Bone River Transect 2: TOCSOM). Analyses of a combined regional dataset of all transects suggest that elevation is the driver of regional diatom variability in Willapa Bay, with salinity and substrate co-varying along an elevation gradient. We identify species with narrow elevation tolerances that can serve as indicator species of specific environments. Despite the site-specific variability of our modern diatom distribution, the regional dataset provides an important dataset that can be used to reconstruct RSL in Willapa Bay.

Bivalve and gastropod shell beds deposited during the Early Pliocene (4.69-5.23 Ma) occur in uplifted outcrops (36 – 180 m above sea level) along the east coast of Patagonian Argentina. These rock units provide a record of sea level during a geologic period when atmospheric CO2 and temperatures were higher than today. As such, reconstructing the elevation of global mean sea level (GMSL) during this time allows us to better understand how sensitive ice sheets are to increased past and future warming. However, reconstructing GMSL from local sea level indicators is hindered by effects such as mantle dynamic topography and glacial isostatic adjustment (GIA) that cause local sea level to deviate from the global mean. Here we use geodynamic modeling to better understand this complex dynamic setting and quantify the amount of uplift along this coastline. Despite being located on a relatively stable passive margin, significant variations in the elevation of the paleo shoreline indicators imply that the underlying convecting mantle is deforming the coastline. In particular, the subduction of the Chile Rise beginning ~18 Ma beneath Patagonia has generated a slab window underneath this region through which hot asthenosphere ascends. However, the former slab is still present deeper in the mantle, which causes a complex interplay between the downwelling slab and the upwelling asthenosphere. To quantify the effects of dynamic topography change since the Pliocene, we run 3D mantle convection simulations using the code ASPECT. We initialize our global model with a composite temperature structure derived from recent tomographic studies and a calibrated parameterization of upper mantle anelasticity. Independent estimates of pressure and temperature from thermobarometric calculations of proximal Pali-Aike xenoliths agree with the thermal structure of the tomography-based Earth model. We back-advect temperature perturbations and extract the resulting change in dynamic topography. Pairing GIA models and a suite of convection simulations in which we vary the viscosity and buoyancy structure with the observed differential paleo shoreline elevations allows us to forward model the most likely scenario for uplift along this coast.

We investigated the mangroves of Southeast Sulawesi, Indonesia, to assess their potential as proxies for reconstructing sea level during the Holocene. Initial investigations confirmed that the mangrove species demonstrate zonations parallel to the shoreline and are dominated by the family Rhizophoraceae with Avicennia, Heritiera and Sonneratia also important constituents of the mangroves.We investigated the vertical distributions of pollen assemblages at three sites. Partial CCA analysis demonstrated that at all three sites, elevation was a significant control on the distribution of pollen assemblages in surface samples. We combined the three contemporary transects to develop a regional transfer function to explain the relationship between the surface assemblages and elevations using the Maximum Likelihood (ML) method. The developed transfer function indicated mangrove pollen can be used as a precise indicator of past sea levels with an error of ±0.22 m. The transfer function was applied to two fossil cores from the Wakatobi Marine National Park and evaluated using the Modern Analogue Technique. Both cores showed similar patterns in changes of palaeo mangrove elevation and had modern analogues in the contemporary training set. We conclude that mangrove pollen is a suitable proxy for reconstructing sea level in tropical environments.

Stable carbon isotopes (δ13C) and the ratio of organic carbon to total nitrogen (C/N) are able to differentiate sources of organic matter that accumulate in coastal depositional sequences, which makes them useful to reconstruct former sea levels. This chapter describes the factors that influence δ13C and C/N geochemistry of coastal wetland sediments, outlines basic methods for its use in relative sea-level reconstruction, and draws attention to several case studies.

In this paper, after a short summary on the processes and rates of bioerosion that contribute to the deepening of a tidal-notch profile, some Mediterranean case-studies are presented, where a tidal notch is prevented either from forming or from being preserved. Furthermore, as shown also in a complementary paper, the recent global sea-level rise is preventing the development of new tidal notches in the present mid-littoral zone. This very useful sea-level indicator, of past temporary standstills of the relative sea level in carbonate rock areas, is of great value in assisting interpretations of relative sea-level change in locations where it is preserved. The possibilities of absence of formation or of preservation, however, imply that it should be interpreted carefully before reconstructing local relative sea-level histories. In particular, the lack of fossil tidal notches cannot be relied upon to interpret the absence of past periods of relative sea-level stabilization. © 2012 Elsevier B.V.

In this paper, after a short summary on the processes and rates of bioerosion that contribute to the deepening of a tidal-notch profile, some Mediterranean case-studies are presented, where a tidal notch is prevented either from forming or from being preserved. Furthermore, as shown also in a complementary paper, the recent global sea-level rise is preventing the development of new tidal notches in the present mid-littoral zone. This very useful sea-level indicator, of past temporary standstills of the relative sea level in carbonate rock areas, is of great value in assisting interpretations of relative sea-level change in locations where it is preserved. The possibilities of absence of formation or of preservation, however, imply that it should be interpreted carefully before reconstructing local relative sea-level histories. In particular, the lack of fossil tidal notches cannot be relied upon to interpret the absence of past periods of relative sea-level stabilization.

Increasing necessity for precision in Holocene sea-level records calls for definitive sea-level indicators to be assessed. These are reviewed by use of examples from the S.W. Pacific, and the technique of pollen analysis of mangrove sediments evaluated as a high resolution sea-level indicator. Example is given how a fully calibrated sea-level reconstruction can be gained from Holocene mangrove peat units from Tongatapu, Tonga, by application of features established from a modern analogue of surface samples to interpret fossil pollen patterns.

In order to determine the height and age of paleo sea level precisely, it is necessary to find suitable indicators. The tube worm Pomatoleios kraussii (BAIRD) was an excellent indicator of sea level stands on the southeast coast of Boso Peninsula. Studies of vertical distribution of P. kraussii, living and emergent at the time of the 1923 Taisho Earthquake, showed that the upper limit of the P. kraussii zone was located at 0.1±0.1m above mean sea level. At Hiraiso, the upper limit of the distribution of fossil P. kraussii emergent at the time of the 1703 Genroku Earthquake was 6.8m. Using the relationship between the upper limit of P. kraussii and mean sea level, paleo sea level prior to the Genroku Earthquake was estimated to be 6.7±0.1m.On dating radiocarbon ages of small samples such as P. kraussii tubes, the alternation of samples might lead to incorrect results. Three out of four radiocarbon ages of fossil P. kraussii tubes emergent at the time of the 1703 Genroku Earthquake were dated to be about 100-250 years older than the time of the earthquake. Observation of these tubes by scanning electron microscopy revealed that the tubes which gave the older ages had been recrystallized, and did not yield reliable ages.

By means of oil capillaries the indicator attenuates the tidal rise and fall of the sea surface, as well as all fluctuations of higher frequencies. The time constant of the response is chosen so that the smaller, slower fluctuation of sea level is transmitted with very little change of amplitude and phase. There is no recorder. Visual reading of the oil level in a burette should be tabulated twice a week. Installation of the instrument is much easier and cheaper than a recording tide gauge.