A 20 ka sediment record from the Hajar Mountain range in N-Oman, and its implication for detecting arid–humid periods on the southeastern Arabian Peninsula

doi:10.1016/j.epsl.2007.10.050

Earth and Planetary Science Letters

Volume 265, Issues 3–4, 30 January 2008, Pages 546-558

https://doi.org/10.1016/j.epsl.2007.10.050 Get rights and content

Abstract

Playa-like sediments from the Hajar Mountain range in northern Oman (22.83°N, 59.00°E; 1050 m asl) document variations of the paleoenvironmental and paleoclimate conditions over the last 20 ka. Based on high-resolution sediment sampling and their OSL dating, sedimentation rates were calculated and used as a proxy for paleorainfall. The results show that the Glacial to Lateglacial was characterized by arid conditions with a following transitional period of even less rainfall. At 10.5 ka, sedimentation rates increases abruptly, indicating the onset of the early Holocene humid period (EHHP). Rainfall reaches its maximum at 9–8 ka (EHHP-2) and a decreasing sedimentation rate after 8 ka characterizes the arid period of the middle to late Holocene. Variations of the hydrological regime are associated with the intensity of the boreal summer Indian monsoon and its related position of the ITCZ. For the onset of the EHHP, a northerly shift of the ITCZ is postulated, thus confirming earlier results from the southern Arabian Peninsula.

Introduction

The Indian Ocean Monsoon (IOM) system is one of the major wind systems on earth. It has a global impact on climate and acts as a prominent conveyer belt of heat and moisture. The IOM dominates the hydrological situation of vast areas in NE-Africa, S-Arabia and India, which has an important impact on the socioeconomic conditions of this region.

The pattern and intensity of the IOM is heavily dependent on the differential heating between the Indian Ocean and the Asian continent. During the boreal summer a low-pressure cell develops over the Tibetan Plateau and a high-pressure cell over the southern Indian Ocean. This causes a northward shift of the Intertropical Convergence Zone (ITCZ) and generates the SW-Monsoon (Fig. 1). Due to the present-day boreal summer position of the ITCZ along the southern coast of the Arabian Peninsula, only the south-coast areas are affected by the moist SW-Monsoon air masses, whereas further inland arid to hyperarid conditions prevail. In the past, the boreal summer position of the ITCZ fluctuated and during times of a northward shift of the ITCZ, moist air masses penetrated further inland, causing increased precipitation. Based on stable isotope analyses, Weyhenmeyer et al. (2000) could demonstrate that the Indian Ocean was the source of these past moist air masses. On a glacial–interglacial timescale, these continental arid and humid periods are thought to be primarily controlled by glacial boundary conditions (Burns et al., 2001), whereas the influence of the northern hemisphere solar insolation on the position of the ITCZ was demonstrated by Neff et al. (2001).

For the late Quaternary several studies show variations in the pattern and intensity of the IOM, documented in various archives from NE-Africa (Crombie et al., 1997, Gasse, 2000), S-Arabia, (Burns et al., 2001, Glennie and Singhvi, 2002, Preusser et al., 2002, Radies et al., 2004), the Arabian Sea (Sirocko et al., 1993, Leuschner and Sirocko, 2000) and India (Andrews et al., 1998, Juyal et al., 2006). The Holocene–Pleistocene transition for example is marked by an intensification of the IOM with the consequence of a strong increase in early Holocene precipitation. For N-Africa this wet period is well known (DeMenocal, 2000) and recorded by extensive lake deposits (Gasse, 2000, Kröpelin and Soulié-Märsche, 1991) and flourishing Neolithic civilizations (Kuper and Kröpelin, 2006). For southwest Arabia, Lézine et al. (2007) identified a period of increased precipitation between 12 ka and 7.5 ka recorded by lacustrine sediments in Yemen. Based on faunal assemblages from interdunal deposits in southern Oman, Radies et al. (2005) identified an early Holocene wet period from 9.3 ka to 5.5 ka. Also in a study from southern Oman but based on speleothems, Fleitmann et al. (2003) reported highly depleted δ¹⁸O values indicating a rapid increase in monsoon precipitation between 10.3 ka and 9.6 ka with a gradual long-term decrease in monsoon precipitation after about 8 ka. Further north at Al Hota cave in the Oman Mountains, a similar onset of the early Holocene pluvial period is indicated by a sudden increase in speleothem growth rates and a drop in calcite δ¹⁸O at 10.5 ka (Burns et al., 2001). However, the wet period ends at Al Hota cave at 6 ka, thus much later than seen by Fleitmann et al. (2003). Two studies from the United Arab Emirates north of Oman, which are based on dune activities (Bray and Stokes, 2004) and lacustrine geochemistry (Parker et al., 2006), identified the end of the wet period also at 6 ka, even if Parker et al. (2006) reported a much later onset of the wet period at 8.5 ka.

To reconstruct the pattern and intensity of the IOM and its associated position of the ITCZ, the Arabian Peninsula represents a key locality. However, as shown by the discussion about the onset and duration of the early Holocene wet period, the timing of climate fluctuations in southern Arabia is still under debate.

Due to a lack of appropriate climate proxies, paleoclimate reconstruction is not easy in dryland areas and often eolian sediments are used as an indicator of past atmospheric conditions (Chase and Thomas, 2007). However, eolian sediments are a good indicator for dry conditions, but bear only indirect information about past fluctuations in precipitation. In this context, fluvially transported sediments represent a more appropriate proxy for precipitation, and even in dryland areas sediments transported by surface run-off can be found under favorable conditions. This is because the fluvial process plays also in dryland areas an important role in sediment redistribution, which arises from a lack of protecting vegetation cover and infrequent but intensive rainfall, allowing surface run-off with the consequence of fluvial sediment erosion, transportation and its corresponding sedimentation. Thus, if water transported sediments are available, they can be used as a proxy for reconstructing paleohydrological conditions (Eitel et al., 2001, Eitel et al., 2005, Eitel et al., 2006, Heine, 2004).

This paper presents results from a playa-like sediment archive near the oasis of Maqta, situated at the upper end of the Wadi Khabbah in the Jabal Bani Jabir mountains (Hajar range) of northern Oman. Based on a 20 m deep sediment profile, the sedimentation history of a sediment-filled depression is reconstructed thereby aiming at shedding some light on the paleoenvironmental and paleoclimate history of the region for the last 20 ka. The chronostratigraphy of the sediment archive was achieved by high-resolution sampling and their optical stimulated luminescence (OSL) dating (Fuchs et al., 2007). Given its unique nature in 2005 the site was revisited to re-sample the sediment profile and to increase the number of OSL samples. While the sediment profile shows a homogeneous structure, marked horizons with rich depositions of mollusk shells and a clear differentiation in the clay contents indicated the possibility to use calculated sedimentation rats and soil chemical properties as a proxy for precipitation in the surrounding catchment area during the late Quaternary period. The main hypothesis of this study therefore was that sedimentation rates of this profile in combination with the soil chemical properties of the profile can be used to deduct precipitation changes as a consequence of fluctuations in the IOM.

Section snippets

Study area

The study site is located 150 m above the mountain oasis of Maqta (22.83°N, 59.00°E; 1,050 m asl), on the southwestern flank of the Hajar range in northern Oman (Fig. 2). Here, an ellipsoid shaped, flat depression (ca. 0.5 km²) is situated on a northwest orientated slope (Fig. 3), with its main catchment (ca. 2 km²) upslope to the southeast, characterized by debris covered steep slopes and composed of Cretaceous and Tertiary limestone. The depression is dominantly filled with fine grain

Methods

Sediment samples from the 20 m deep pit were taken from 9 pm to 2 am during two moon-less nights using a rope-ladder of aluminum steps and steel ropes. The dominantly homogeneous sediment profile was continuously sampled at 0.5 m intervals, whereas sampling intervals were adjusted to changing sediment structures in the middle section of the profile. For OSL sampling the profile was carefully cleaned and the samples were directly taken from the pit wall using a scraper and black, light-proof

Sediment analysis

Based on the field observations and analytical data, three main sedimentary units were distinguished (Fig. 4 and Table 1). The lower part of the section (unit 3) from 20 m to 12 m is characterized by a homogenous light brown sediment (7.5YR6.3; Munsell Color Corporation 1976) with a calcium carbonate content (CaCO₃) of 42–48% and a SiO₂ content of the carbonate-free sediment fraction of 25–36%. Because of a change in the clay content in favor of the sand fraction at ca. 14 m depth, unit 3 is

Glacial to Lateglacial

Based on the OSL ages and the calculated sedimentation rates, paleorainfall and the overall paleoenvironmental conditions seemed to be homogeneous in the period from 19.16 ± 1.39 ka to 12.50 ± 0.92 ka. In this period of sediment unit 3b, paleorainfall is interpreted to be low, which is supported by low values of C_org as a proxy for the remains of below ground biomass production (low root biomass reflecting low shoot biomass) and high values of CaCO₃ as a proxy for low solution and vertical

Conclusions

Based on OSL ages, the calculated sedimentation rates from the sediment record from Maqta in northern Oman represent a valuable proxy for paleorainfall of the last 20 ka, which was confirmed by additional chemical analyses of the sediments. The key inferences are:

(1)
The Glacial to Lateglacial period seems to be characterized by a homogeneous sedimentation rate, lasting from 9.16 ka to 12.50 ka. During this period, rainfall was low, indicating arid conditions.
(2)
The following transitional period from

Acknowledgements

We would like to thank Manfred Fischer from the University of Bayreuth for luminescence sample preparation, and Detlef Degering from the Saxonian Academy of Science for low-level γ-spectrometry measurements. Hans von Suchodoletz from University of Bayreuth is thanked for the semi-quantitative quartz analysis, Eva Wiegard and Claudia Thieme from University of Kassel-Witzenhausen for soil chemical analyses and Eike Luedeling for constructing the topographic model of the northern Oman mountains in

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A 20 ka sediment record from the Hajar Mountain range in N-Oman, and its implication for detecting arid–humid periods on the southeastern Arabian Peninsula

Abstract

Introduction

Section snippets

Study area

Methods

Sediment analysis

Glacial to Lateglacial

Conclusions

Acknowledgements

Quat. Res.

Geomorphology

Quat. Int.

Palaeogeogr. Palaeoclimatol. Palaeoecol.

Quat. Sci. Rev.

Quat. Int.

Geomorphology

Palaeogeogr. Palaeoclimatol. Palaeoecol.

Quat. Sci. Rev.

Quat. Sci. Rev.

Quat. Sci. Rev.

Quat. Geochronol.

Quat. Sci. Rev.

Earth Planet. Sci. Lett.

Quat. Sci. Rev.

Quat. Sci. Rev.

Quat. Res.

Catena

Quat. Sci. Rev.

Palaeogeogr. Palaeoclimatol. Palaeoecol.

Radiat. Meas.