A climatic evaluation of the southern dispersal route during MIS 5e.
Samuel L. Nicholson1,2, Rob
Hosfield2, Huw S. Groucutt3,4,5,
Alistair W. G. Pike6, Stephen J.
Burns7, Albert Matter8, and Dominik
Fleitmann2,9
1 Ecology and Evolutionary Biology, University of
Reading, Reading, RG6 6LA, United Kingdom.
2 Department of Archaeology, University of Reading,
Reading, RG6 6AB, United Kingdom.
3 Extreme Events Research Group, Max Planck Institute
for Chemical Ecology, 07745 Jena, Jena, Germany.
4 Department of Archaeology, Max Plank Institutes for
Chemical Ecology, the Science of Human History, and Biogeochemsitry,
07745 Jena, Germany.
5 Institute of Prehistoric Archaeology, University of
Cologne, Cologne, 5093, Germany.
6 Department of Archaeology, University of
Southampton, Southampton, SO17 1BF, United Kingdom.
7 Department of Geosciences, University of
Massachusetts, MA 01003-9297, United States of America.
8 Institute of Geophysics, University of Bern, CH-3012
Bern, Switzerland.
9 Department of Environmental Sciences, University of
Basel, CH-4056 Basel, Switzerland.
Corresponding authors: Samuel L. Nicholson
(sam.nicholson@reaeding.ac.uk)
Dominik Fleitmann
(dominik.fleitmann@unibas.ch)
Highlights:
- Timing of monsoon intensification in Arabia confined to 127.7 ka BP
until 121.1 ka BP.
- Most substantial increase of rainfall in the last 130 ka BP.
- Onset of the pluvial period lagged sea-level rise.
Abstract
Homo sapiens dispersals out of Africa are often linked to
intensifications of the African Summer Monsoon and Indian Summer
Monsoon. Current dispersal models advocate that dispersals along the
“southern-route” into Arabia occurred during Glacial Termination-II
(T-II), when reduced sea-level and Bab-al-Mandab width increased the
likelihood of crossing. The precise phasing between sea-level and
monsoon precipitation is thus key to assess the likelihood of a
successful crossing or the behavioural and technological capacities that
facilitated crossing. Based on a precisely-dated stalagmite record from
Yemen we reveal a distinct phase-lag of several thousand years between
sea-level rise and monsoon intensification. Pluvial conditions in
Southern Arabia during MIS 5e lasted from ~127.7 to
~121.1 ka BP and occurred when sea-levels were already
higher than at present. Based on our observations, we propose three
models for the dispersal of H. sapiens which all have pertinent
implications for our understanding of human technological and
behavioural capacities during MIS 5e.
1 Introduction
Understanding how H. sapiens spread from Africa across the world
is one of the most debated topics in human evolution (Mellars et al.,
2013; Groucutt et al., 2015a; Bae et al., 2017). Two proposed main
dispersal routes cross Arabia: a northern-route across the Sinai into
the Levant and a southern-route from the Horn of Africa via the Strait
of Bab-al-Mandab into Southern Arabia and beyond (Fig. 1). The
accessibility of these entry points was spatiotemporally variable and
related to major climatic changes across the Saharo-Arabian deserts.
During interglacial periods, both the African and Indian Summer Monsoons
(ASM and ISM, respectively) were much stronger, expanded northward and
transformed the Saharo-Arabian deserts into green landscapes for a few
millennia (Fleitmann et al., 2003b; Parton et al., 2015; Petraglia et
al., 2015; Tierney et al., 2017; Nicholson et al., 2020). These pluvial
periods, termed “Green Arabia Periods” and “South Arabian Humid
Periods” (SAHPs) respectively, provided optimal periods for H.
sapiens to disperse from sub-Saharan Africa into Eurasia (Fleitmann et
al., 2011; Rosenberg et al., 2011; Larrasoaña et al., 2013; Nicholson et
al., 2020). Over the last 130 ka BP, pluvial conditions in Southern
Arabia with rainfall of more than 300 mm yr-1 occurred
during Marine Isotope Stages (MIS) 5 and 1, and lasted from
~128-121 ka BP (MIS 5e; SAHP 4), ~104-97
ka BP (MIS 5c; SAHP 3) and ~84-71 ka BP (MIS 5a; SAHP 2)
and ~10.5 to 6.2 ka BP (SAHP 1) (Fleitmann et al., 2011;
Nicholson et al., 2020). In addition, there is also some evidence for a
period of enhanced rainfall between approximately 60 and 50 ka BP (the
onset of MIS 3) (McLaren et al., 2009; Parton et al., 2013, 2018),
though the nature and timing of this period remains uncertain.
The southern dispersal route involves a maritime crossing of the
Bab-al-Mandab Strait. However, its current width of approximately
~26 km represents a significant challenge to dispersal
and was more likely traversable at times of lower sea-level, especially
if sea-faring technologies were limited. One proposed timing for earlyH. sapiens dispersals is Glacial Termination-II (T-II), between
136-129 ka BP, when sea-levels, although rapidly rising, were lower than
today and the width of the Bab-al-Mandab Strait
(BaMwidth) was reduced to a few kilometres (Armitage et
al., 2011). From a palaeoclimatic perspective, a dispersal was most
likely to have occurred at times of increased precipitation and biomass
across Arabia. However, during T-II, several lines of evidence point to
a phase-lag of several thousand years between sea-level rise and
northward migration of the tropical rainbelt due to colder
northern-hemisphere temperatures related to Heinrich Stadial (HS) 11
between 135 and 130 ka BP (Cheng et al., 2009; Böhm et al., 2015;
Häuselmann et al., 2015; Marino et al., 2015). In other words, arid
conditions may have prevailed in Arabia during T-II, forming a
biogeographical barrier to widespread dispersals despite low sea-levels.
Thus, establishing the precise phasing between sea-level change and
ASM/ISM intensification during the MIS 6-5e transition from records
close to the Bab-al-Mandab Strait could be one critical factor for
understanding accessibility of the southern-dispersal route. ASM and ISM
records with precise and accurate chronologies are an important
prerequisite to reveal such a phasing. Here, we present a
precisely-dated and highly-resolved speleothem-based climate record from
Mukalla Cave in Yemen, covering MIS 5e (SAHP 4: Nicholson et al., 2020).
Precise Uranium-series (230Th) ages allow us to
evaluate the temporal phasing between ASM/ISM rainfall and sea-level
change at a possible point of entry into southern Arabia.
2 Environmental Settings, materials and methods
Stalagmite Y99 was collected from Mukalla Cave (14°55’02”N; 48°35’23”
E; ~1500 masl; Fig. 1) in southern Yemen, where climate
is strongly governed by the ASM and ISM respectively. At present, both
Mukalla Cave and the Bab-al-Mandab Strait are located at the northern
and north-eastern margins of the ASM and ISM, with rainfall averaging
<150 mm yr-1 (Fleitmann et al., 2011).
Stalagmite Y99 extends back to 1.1 million years and was deposited in 17
punctuated growth intervals identified through 230Th
and Uranium-lead dating, with Growth Interval-I (GI-I) being the
youngest and dated to MIS 5e (Nicholson et al., 2020). Previously
analysis of Y99 was focussed on the broad timing and climatic conditions
(bulked δ18O and δ13C isotope
analysis) of SAHPs over the last 1.1 million-years (Fleitmann et al.,
2011; Nicholson et al., 2020). Here, we provide a more high-resolution
and focussed study of the timing of SAHP 4 compared to sea-level
fluctuation. We used the StalAge algorithm to produce a robust age-model
for SAHP 4 from previously collected 230Th ages. This
was then used to provide δ18Oca (ASM
rainfall) and δ13Cca records at
<100 years resolution and can be accurately compared to
sea-level records.
The width of the Bab-al-Mandab Strait (BaMwidth) was
reconstructed using bathymetry data and the Red Sea relative sea-level
(RSL) curve. The RSL has been constructed using marine core
δ18OG. ruber records from the Red Sea
(Siddall et al., 2003; Rohling et al., 2009), whereas the chronology of
the RSL time-series is based on correlations with Mediterranean Core
LC21 and the revised 230Th chronology of the Soreq
Cave record for periods younger than 150 ka BP (Rohling et al., 2009;
Grant et al., 2012, 2014). Using local sea-level records that exploit a
basin isolation effect means that our assessment is unaffected by
isostatic effects, allowing us to compare regional climates with
sea-level variations that control the sill depth and the width of the
Strait. We used the freely available QGIS software package and 15
arc-second (~450 m between 12-14oN)
interval elevation and bathymetry data (GEBCO Compilation Group, 2020)
in combination with the RSL to estimate BaMwidth over
the last 150 kyrs (extended methods).
3 Results and discussion