Restoration of paleo-water level of Lake Biwa during Holocene

利用5个钻孔岩心对琵琶湖周边湖泊的沉积过程和环境史进行了研究。结果如下。1)内湖有两种类型。一个是封闭的内湖，在山的后面，因此泥沙供应不足。另一种类型为开阔内湖，位于三角洲，间歇补给充足的泥沙。2)封闭内湖沉积物由连续的腐殖质土壤和泥炭组成，含Kikai-Akahoya tephra（约7.3ka）。这意味着稳定的沼泽或浅水环境在全新世晚期的封闭泻湖中持续存在。开阔的内湖沉积物由砂、泥和砾石蚀变组成，含薄腐殖质层。这些沉积表明河流平原和三角洲的环境变化频繁。3)在一些封闭内湖沉积物的上部推断出无机沉积物，并确定了历史年龄。这表明人类活动如围垦等影响了内湖周围低地的无机沉积物产物。4)与琵琶湖西侧的南沼和东侧的松沼两个封闭内湖矿床相比，相同的基准层位在南沼岩心处比在松沼岩心处有系统的深，而这两个矿床都是在约4ka层位下由含真草根的泥炭组成。说明琵琶湖西侧活动断裂在约4ka以前就有活动，琵琶湖西侧发生了沉降

We investigated depositional process and environmental history of inner lakes around Lake Biwa, by using of five boring cores.The results are follows. 1) There are two types of inner lakes. One is the closed inner lake which bates behind the mountains, so sediment supply is not enough. The other type is open inner lake which locates in the delta, enough sediment are supplied intermittently. 2) Closed inner lake deposits consist of continuous humic soil and peat, containing Kikai-Akahoya tephra (c.a. 7.3ka). This means stable marshy or shallow water environments had been continued during late Holocene in the closed lagoon. The open inner lake deposits consist of alteration of sand and mud and gravels, containing thin humic layers. Such deposits indicate frequent environmental changes in the fluvial plain and delta. 3) Inorganic sediments are concluded in the uppermost part of some closed inner lake deposits which dated historical age. This suggests that human activities such as reclamation on the lowland surrounding the inner lake affected product of inorganic sediments. 4) In comparison with two closed inner lake deposits in the western side and eastern side of Lake Biwa, Minami-numa located western side and Sone-numa located eastern side, the same datum horizons are systematically deeper at the Minami-numa core than at the Sone-numa core, while both two deposits consist of peat containing rootlets of true grasses under c.a.4ka horizon. This means active faults near the western side of Lake Biwa acted before c.a.4ka, and western side of Lake Biwa subsided