非洲被認為和人類的起源有很大的關係，而瞭解非洲古氣候變化對於研究人類發展有很大的幫助，許多非洲的前人研究古氣候的時間尺度都是百萬年為單位或是集中在晚更新世至現代的範圍，對於過去10-30萬年的東非古氣候記錄還甚為缺乏，為了研究此時期的古氣候變遷，我們首先需要一個材料，這個材料必須可以提供準確的定年，以及保存良好的古氣候記錄。
在東非肯亞境內的馬加地湖為一鹼性湖泊，前人研究發現，當氣候較濕潤時湖泊內會沉積燧石，在馬加地湖採集到的岩芯中發現了大量的燧石，因其堅硬的特性、分佈的深度範圍廣泛，非常適合作為研究古氣候的定年材料。本研究使用東非肯亞馬加地湖泊兩根岩芯 (HSPDP-MAG14-1A、HSPDP-MAG14-2A) 的19個燧石樣本來作精確的鈾系同位素量測及訂定年齡，其年齡分佈約在過去10-30萬年。前人的研究中還鮮少使用燧石作為鈾系同位素定年的材料，因此，本研究著重於探討燧石鈾系同位素定年的潛力，其中也探討了鈾系同位素定年法在燧石樣本應用的條件及方法，並且更進一步的利用鈾釷同位素探討馬加地湖的古氣候變遷。結果顯示除了其中三個樣品可能混入二次結晶的礦物之外，大部分的燧石樣品都能提供穩定且可靠的定年，因此在樣品的挑選上必須格外小心。岩芯沉積速率及各個樣品中的活度比值234U/238U、活度比值238U/232Th顯示在過去10-30多萬年的期間內，MIS6和MIS8有濕潤期的記錄，MIS6和MIS8為兩個冰期，和高緯度地區的研究中，冰期為乾旱氣候的記錄不同，推測其主要原因為地球軌道變化使太陽直射地球的角度和地點的改變，使得熱帶輻合帶 (Intertropical Convergence Zone, ITCZ) 和剛果大氣界線 (Congo Air Boundary, CAB) 較常停留在東非赤道地區，進而讓此地區在冰期時保持了一定的降雨量，而有濕潤氣候的記錄。雖然本研究無法證明這些氣候事件與人類發展的直接關聯，但在人類發展歷史上，過去20萬年前也就是MIS6的時期人類演化成了最接近現代類的人種，智人 (Homo sapiens)，而我們發現這個時期在東非是較濕潤的氣候。另外，本研究也發現馬加地湖從MIS5開始漸漸變得乾旱，這也許導致了在過去約10萬年時人類的遷徙，離開非洲。

Summary
The paleoclimate of east Africa played a very important role in the human development history. To realize it, we need a systematic and accurate dating. The chert of Lake Magadi was formed from an alkline igneous lake with hot spring. Chert is so hard that it can keep the co-precipitation uranium from losing. This property provide an assurance of reliability of U-Th dating. This study focus on the potential of chert on uranium isotope dating and the paleoclimate in east Africa. We performed uranium-series isotope dating of nineteen chert samples collected from various depths of two sediment cores (HSPDP-MAG14-1A, HSPDP-MAG14-2A) drilled from the lake bed of Lake Magadi, southern Kenya. The results show that most of samples could provide a reliable age by U-Th dating. Some samples are not consistent with the stratigraphic sequence (1A-33Y-1, 1A-51Y-2, 2A-30Y-2). We test these samples and the samples under or above them again. As a conseqence, we infer that these three samples contain some re-crystallization minerals. We also can find some trends in the isotope activity ratio 234U/238U, 238U/232Th and the sedimentary of core in Lake Magadi. All of them show that there were humid records in MIS6 and MIS8. This result is different from the study in high latitude region before. We consider that it might relate to the change of the path of Intertropical Convergence Zone and Congo Air Boundary in the glacial period.

Introduction
Over the last decade , previous studies indicated that east Africa was the origin of Homini. An understanding of the paleoenvironmental changes as recorded in its lake sediments bears significant implications for archaeology, anthropology, regional geology, and global climatology. Many studies on east Africa paleoclimate on long time scale have been done, but the results for early to middle Pleistocene still didn’t reach a consensus. We coorporated with the organization of the project: HSPDP (The Hominin Sites and Paleolakes Drilling Project), which consists of more than one hundred scientists, eleven contries. The main goal is to address evolution of Homo sapiens by the paleoclimate of east Africa. Eighteen cores were drilled at Ethopia and Kenya. One of drilling site was lake Magadi, it’s located in south Kenya. Abundant cherts deposited around two cores (HSPDP-MAG14-1A, HSPDP-MAG14-2A) in lake Magadi. We dated chert by U-Th dating techniques to reveal the regional paleoclimate.

Materials and Methods
Ten cherts were sampled from HSPDP-MAG14-1A and nine cherts from HSPDP-MAG14-2A. The experiment contained two parts. In the first part, we separated each five samples (2A-21Y-1, 2A-32Y-1, 2A-42Y-1, 2A-50Y-1, 1A-56Y-1A) into three subsample, cleaned by DI water and ultrasonicator. About 0.6 g samples was dissolved in HNO3-HF-HClO4, a mixed 228Th–229Th–232U spike was added. The digested solutions were dried on the hotplate. U and Th were then extracted and purified by anion exchange methods (Bio-Rad AG1X8, 100-200 mesh). The purified U, Th were measured by α counting. In the second part, we cleaned all nineteen chert samples with nitrogen gun. About 0.02 g samples dissolved in HNO3-HF-HClO4, a mixed 229Th–233U–236U spike was added and dried on the hotplate. U and Th were purified by anion exchange methods (Spetrum IE 1X8 Resin 100-200 mesh), isotopic measurement followed by MC-ICP-MS.

Results and Discussion
U-Th isotopic compositions and the dating results show that the three sub-samples of each sample formed at the same age except 2A-21Y-1 in the first part experiment. It represent most of chert samples can provide a reliable uranium isotope dating. The results of second part experiment exhibit that most obtained cherts dates are consistent with the stratigraphic sequence, except 1A-33Y-1, 1A-51Y-2 and 2A-30Y-2. We estimated these three samples and the sample which was upper or lower to them again. As a consequence, the results shows that 1A-51Y-2 and 2A-30Y-2 are still inconsistent with the stratigraphic seqence and the date of 1A-33Y-1 is different from first part experiment. We surmise that these samples were effected by re-crystallization minerals. All cherts were formed in 50-280 (ky B.P.).The characteristic of sample 2A-21Y-1 is different from other eighteen samples. It has many holes on its surface. We found inconsistent dating results for subsamples of this sample. To verify the above assumptions, we crushed this sample again and saparated it into two parts. One of it cleaned with DI water, another one with alcohol. After measured by MC-ICP-MS, the dates show that both two parts are 40 (ky). Apparently trona in the sample was effectively removed, 40 (ky) is the most reliable date for sample 2A-21Y-1.
The U-Th concentration of cherts correspond to the paleoclimate of Lake Magadi in the past. According to the sedimentary of core, 234U/238U and 238U/232Th activity ratio, they indicates that there was a humid period at MIS6 and MIS8 in lake Magadi. MIS6 and MIS8 were glacial period. This result is different from previous studies in high latitude regions, which performed the drier record during the glacial period. We infer that the mechanism of climate in the African equatorial region has a great impact on this result. Rainfull of east Africa region was dominated by ITCZ (Intertropical Convergence Zone) and CAB (Congo Air Boundary). The movement of ITCZ and CAB could follow insolation change. The insolation of north hemisphere may decrease and equator region would keep stable at the glacial period. It might cause ITCZ and CAB moved to equator region of east Africa and bring stable rainfull at the glacial period, MIS6 and MIS8 in Lake Magadi. On the other hand, we also suggest that lake Magadi has gotten more and more arid since MIS5. It might cause the Homo sapiens migrate out of Africa in the past 100 (ky).

Conclusion
The U-series results obtained on Lake Magadi chert clearly indicate that chert can be reliably dated by U-Th dating, but we should be cautious in sample selection. U isotope systematic exhibits that there were humid records in MIS6 and MIS8, two glacial period. Nevertheless, according to predecessors literature, the paleoclimate reseaches which focused on MIS6 to MIS8 were few, but there was a humid record in LGM (Last Glacial Maximum) in east Africa. It’s caused by the relation between insolation, ITCZ and CAB which was described in Results and Discussion. The implications on the relation of paleoclimate and the evolution of Homini are: 1. Homo sapiens appeared in east Africa in past 200 (ky) may related to MIS6 was a humid period. 2. East Africa was more and more arid may caused Homo sapiens migrated out of Africa.

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