Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA.
Proc Natl Acad Sci U S A. 2012 Jun 26;109(26):E1688-94. doi: 10.1073/pnas.1112743109. Epub 2012 May 29.
The collapse of the Bronze Age Harappan, one of the earliest urban civilizations, remains an enigma. Urbanism flourished in the western region of the Indo-Gangetic Plain for approximately 600 y, but since approximately 3,900 y ago, the total settled area and settlement sizes declined, many sites were abandoned, and a significant shift in site numbers and density towards the east is recorded. We report morphologic and chronologic evidence indicating that fluvial landscapes in Harappan territory became remarkably stable during the late Holocene as aridification intensified in the region after approximately 5,000 BP. Upstream on the alluvial plain, the large Himalayan rivers in Punjab stopped incising, while downstream, sedimentation slowed on the distinctive mega-fluvial ridge, which the Indus built in Sindh. This fluvial quiescence suggests a gradual decrease in flood intensity that probably stimulated intensive agriculture initially and encouraged urbanization around 4,500 BP. However, further decline in monsoon precipitation led to conditions adverse to both inundation- and rain-based farming. Contrary to earlier assumptions that a large glacier-fed Himalayan river, identified by some with the mythical Sarasvati, watered the Harappan heartland on the interfluve between the Indus and Ganges basins, we show that only monsoonal-fed rivers were active there during the Holocene. As the monsoon weakened, monsoonal rivers gradually dried or became seasonal, affecting habitability along their courses. Hydroclimatic stress increased the vulnerability of agricultural production supporting Harappan urbanism, leading to settlement downsizing, diversification of crops, and a drastic increase in settlements in the moister monsoon regions of the upper Punjab, Haryana, and Uttar Pradesh.
青铜时代哈拉帕文明的崩溃一直是个谜。城市主义在印度河流域平原的西部地区繁荣了大约 600 年，但大约从 3900 年前开始，定居总面积和定居点规模缩小，许多遗址被废弃，遗址数量和密度向东显著转移。我们报告了形态和年代学证据，表明在该地区大约 5000 年前的干旱化加剧后，全新世晚期哈拉潘地区的河流景观变得异常稳定。在旁遮普的冲积平原上游，旁遮普的大型喜马拉雅河流停止了侵蚀，而在下游，印度河在信德建造的独特的巨型河流脊上的沉积物沉降速度减缓。这种河流的平静表明洪水强度逐渐减弱，这可能最初刺激了 intensive agriculture，并鼓励了大约 4500 年前的城市化。然而，季风降水的进一步减少导致了既不利于 flood-based 农业又不利于 rain-based 农业的条件。与之前的假设相反，一些人认为，一条由喜马拉雅山冰川补给的大河，即神话中的 Sarasvati，为印度河和恒河盆地之间的河间地的哈拉潘中心地带提供了水源，我们表明，在全新世期间，只有季风河流在那里活跃。随着季风的减弱，季风河流逐渐干涸或成为季节性河流，影响了其流经地区的宜居性。水文气候压力增加了支持哈拉潘城市主义的农业生产的脆弱性，导致定居点规模缩小、作物多样化以及上旁遮普、哈里亚纳邦和北方邦较湿润季风地区的定居点急剧增加。
