Early Holocene greening of the Sahara has been inferred from many sedimentary archives (e.g. Hoelzmann et al., 2001). Likewise, over the last two decades similar reconstructions of lakes and a more humid climate have been established for the southern Arabian Peninsula (e.g. Fleitmann et al., 2007; Engel et al., 2017) and the Levant (Bar-Matthews et al., 2003). Such evidence also exists for northern Arabia (Schulz and Whitney, 1986; Crassard et al., 2013; Zielhofer et al., 2018), but is limited in sufficiently robust proxy data and chronological resolution, hampering our understanding of the scarce archaeological record of that time (Hilbert et al., 2014). In this paper, we present latest results of the ongoing DFG-funded project CLEAR, which explores the highly resolved palaeolake record of the sabkha basin in the oasis of Tayma, northern Arabia. Today only flooded episodically after rainfall events, the endorheic basin is encircled by a ring of isolated shoreline deposits in an altitudinal corridor of only a few metres, consisting almost entirely of Melanoides tuberculatus and Hydrobia sp. shells, Amphibalanus amphitrite carapaces, foraminifers, and ostracods, with minor amounts of siliciclastic sand (Engel et al., 2012; Pint et al., 2017). These deposits have recently been mapped and dated by 14C and OSL, and indicate the presence of an early Holocene lake with a depth of up to 17 m and an area of up to 22 km². They correlate with partly varved lake sediments of the central basin according to the 14C-(pollen concentrates), varve- and cryptotephra-based chronology (Dinies et al., 2015; Neugebauer et al., 2017). In the framework of CLEAR, the palaeolake sequence was subjected to detailed sedimentological, geochemical and micropalaeontological analyses (grain-size distribution, XRD, µXRF, thin- section studies, foraminifera, ostracods, diatoms, pollen, stable isotopes, C/N, lipid biomarkers). Current results indicate increasing moisture at Tayma from c. 9300 cal. yrs. BP with pronounced humid conditions only over the second half of the 9th millennium BP, represented by an annually varved sequence of aragonite-, diatom-, and clastic silt- dominated laminae. After 7950 cal. yrs. BP, aridification set in, leading to sabkha development at c. 4200 cal. yrs. BP and the accumulation of aeolian sand. The rather short period of increased moisture availability contrasts with adjacent records from southern Arabia and the Levantine region (Bar-Matthews et al., 2003; Fleitmann et al., 2007), which reflect more humid conditions over several millennia during the early to mid-Holocene. This is a contribution to the research project “CLEAR – Holocene Climatic Events of Northern Arabia” (DFG PL 535/2-1; FR 1489/5-1; EN 977/2-1); see also contribution Pint et al. (this conference) and project website https://clear2018.wordpress.com. References: Bar-Matthews, M, et al., Geochim. Cosmochim. Acta 67, 3181–3199 (2003); Crassard, R., et al., PLOS ONE 8, e68061 (2013); Dinies, M., et al., Quat. Int. 382, 293–302 (2015); Engel, M., et al., 2012, Quat. Int. 266, 131–141 (2012); Engel, M., et al., Global Planet. Change 148, 258–267 (2017); Fleitmann, D., et al., Quat. Sci. Rev. 26, 170–188 (2007); Hilbert, Y.H., et al., J. Archaeol. Sci. 50, 460–474 (2014); Hoelzmann, P., et al., Palaeogeogr. Palaeocl. 169, 193–217 (2001); Neugebauer, I., et al., Quat. Sci. Rev. 170 269–275 (2017); Pint, A., et al., J. Foram. Res. 42, 175–187 (2017); Schulz, E., Whitney, Hydrobiologia 143, 175–190 (1986); Zielhofer, C., et al., Quat. Int. 473, 120–140 (2018).
Located in
Library
/
RBINS Staff Publications 2018
Coasts around the world are affected by high-energy wave events like storm surges or tsunamis depending on their regional climatological and geological settings. Coarse clasts (boulders to fine blocks) deposited on the shore can provide evidence for hazard-prone areas and physical characteristics of the flooding event. In order to better understand the process of boulder transport by tsunamis and to calibrate numerical hydrodynamic models, we conducted physical boulder transport experiments in a Froude-Scale of 1:50 utilizing idealized boulder shapes (cuboids) as well as realistic, complex boulder shapes based on real-world data. Comparing the behaviour of natural shaped with idealized boulders, allows identifying how the boulder shape influences the transport process in terms of transport mode (sliding, shifting, saltation), path and distance. Experiments are conducted in a 33 m long and 1 m wide flat wave flume ending on an ascending coastal profile. The gradient angle of the ramp changes from 11◦ to 4◦ ending on a flat elevated platform resulting in a total length of 4.5 m. The complex shaped boulder model (17.4x9.6x7.6 cm3) is constructed from photogrammetric data of a coastal boulder on Bonaire in the Dutch Caribbean (BOL2 in Engel and May, 2012), which is assumed to be transported by a tsunami. A cuboid boulder model of equivalent volume and weight (14x8x6 cm3) is created for comparison. The tsunami is modelled as a broken bore generated by two computer-controlled pumps. Each experimental run set-up was repeated for at least three times. The results show a significant influence of the boulder shape, in particular regarding the area of the contact surface when the bore approaches the boulder. With increasing contact surface higher transport distances occur. Due to the shape of the complex boulder tends slightly towards a rough ovoid, which is more streamlined than the idealized shape, the effectively acting drag force decreases and leads to reduced transport distances. The predominant transport mode during the experiments was sliding combined with gentle rotating around the vertical axis. However, in several experimental cases the complex boulder significantly rotates while the idealized does not. Recognizing that the transport distance, presumably due to decreasing ground contact and therefore less friction, increases during rotational transport, it is remarkable that the complex boulder still does not reach the transport distances of the idealized one. Experiments for boulder-boulder interactions generally show reduced transport distances. The bore-facing boulder generates a “flow shield” preserving the latter boulder from movement. In consequence, the bore-facing boulder hits its neighbour and stops moving. Within the range of our experiments, this boulder-boulder impact does not exceed a necessary energy-threshold for dislocating the second boulder. Beside further results regarding the influence of the initial water level, increased bottom friction and exper- iment sensitivity, insights into a numerical model based on these experiments will be presented. Engel, M.; May, S.M.: Bonaire’s boulder fields revisited: evidence for Holocene tsunami impact on the Leeward, Antilles. Quaternary Science Reviews 54, 126–141, 2012.
Located in
Library
/
RBINS Staff Publications 2018
