美國加州聖塔芭芭拉地區海洋岩芯之地球化學研究

Abstract

本研究以取自美國加州聖塔芭芭拉海盆長51公分的重力岩心SBB-8-2012(34o 17'N, 120o 03'W，深度約580米)作為地球化學研究的材料，研究沉積物中有機碳14C活度變化和元素含量變化的原因，對該海盆的有機碳和沉積物來源以及水團混合進行討論，從而了解自然和人類對研究地區的影響。岩芯的年代利用三種定年手法:紋層計數、210Pb定年、14C定年。透過210Pb結果給出岩芯的沉積速率為0.25~0.29 cm/year，求得此岩芯底部年代為西元1815年，與紋層計數年齡相當。但是，沉積物總有機碳14C的定年結果在500~4600 yr BP之間，顯示受到海洋碳庫中老碳的影響，因此不能為定年依據。岩心中有機碳含量高（X-ray通光較強的層位）陸源物質輸入少（Si, K, Ti低），14CTOC年齡受老碳污染小，較年輕。海盆內碳庫老碳來源受控於陸源沉積物的輸入，海洋環流的變化、生物作用以及海盆內老碳再礦化，這些因素受到海盆沉積環境變化影響，使得沉積物中的有機碳年代改變，造成14CTOC定年的波動。 海盆沉積環境變化影響除了影響14CTOC年齡的波動，在Itrax XRF core Scanner全岩相與ICP-OES稀酸可溶相元素分析結果也呈現高低變化。結合Δ14CTOC與化學元素的變化，岩芯沉積階段大致分為三個時期: (I) 1870~1815年: 沉積紋層不太明顯，XRF分析元素Ca/Ti和Fe/Mn比值都不高，但Si, K和Ti含量較高，顯示為陸源輸入主要的沉積物。稀酸可溶相中Ca、Sr海相元素濃度較高，其他陸相元素濃度則較低，表明在海洋自生礦物相和吸附相由河水輸入帶來的陸相元素較少。岩芯沉積物中有機碳14C年齡偏老是因為陸源輸入的POC帶入老碳；陸源輸入的DOC被氧化混入海水DIC而帶入老碳；海盆古老沉積物中的有機碳被還原成CH4釋放到孔隙水和海水中，再被氧化為CO2與海水中的DIC混合，而海水中生成的有機碳利用DIC，使得新沉積的有機碳14C年齡偏老。在這部分，老碳的影響造成沉積物有機碳14C年齡比真實的沉積年齡偏老大約1500~2500年。而在某些時期由於老碳輸入增加，造成初始14CTOC年齡偏老可達4500年。 (II)1950~1870年，XRF結果顯示Fe/Mn和Ca/Ti比值都增加，沉積物仍以陸源沉積物輸入為主，但Si, K和Ti的含量減低，顯示陸源輸入減少。沉積物中顯示有一些鈣質殼類形成，鐵質氧化物增加（Fe含量增加），富含有機質的紋層增加，顯示這段時間海洋生產力增加，氧化條件增加，還原條件減低，但在稀酸可溶相元素中各元素含量卻相對減少，表明由於取樣方法的差異，使相同元素在不同分析方法呈現不同的變化。陸源輸入減少反映在稀酸可溶相元素濃度上，推測0.5NHCl所溶出之陸源礦物相對較多。在此一時期，Δ14CTOC波動幅度從下至上逐漸減小，老碳的影響逐漸降低，除底部一個年齡超過2000年，所有年齡都小於2000年，上部年齡都在1000年以內。 (III) 2012~1950年，沉積紋層明顯，XRF分析元素Si、K、Ti含量增加，變幅增大，Ca減少，Fe/Mn比值較低，顯示為還原環境增強。沉積物中的稀酸可溶相元素在不同紋層中變化很大，元素變化與X光照片穿透程度變化相關。在X光穿透程度較高的地方，全岩相及稀酸可溶相元素含量都相對較低，且對比年代推測受聖嬰及反聖嬰現象影響。反聖嬰時期，稀酸可溶相元素含量降低，聖嬰時期增高。岩心沉積物有機碳14C年齡在這個時段最小年齡變小，這是因為1950年之後的核爆使得海水中DIC的14C活度升高，造成老碳對初始年齡的影響較小。同時，海盆的還原條件增強，使得CH4不易被氧化，老碳影響減弱。Δ14CTOC波動也同元素含量波動一樣呈現類似聖嬰及反聖嬰影響的現象。

A 51-cm gravity core, SBB-8-2012, was collected from 580 m water depth in Santa Barbara Basin (SBB) of California (34o 17'N, 120o 03'W) in 2012. We use lamination counting by Itrax XRF core scanner, alpha spectrometer 210Pb dating and AMS 14C dating to establish the chronology of this core. The 14C age of the core materials cannot represent the true depositional ages because of marine reservoir age effect. The 210Pb dating results yield a linear sedimentation rate of 0.29 cm/year for the top 15-cm and a linear sedimentation rate of 0.25 cm/year below 15-cm of the core, which are similar to the lamination counting. The core age is from AD 2012 on the top to AD 1815 at the bottom spanning 197 years’ sedimentary history. SBB marine reservoir age for the past 200 years is controlled by variations of the old carbon sources into the sediments and the 14C activity of the atmospheric CO2. The old carbon sources in SBB are the input of terrigenous sediments, the changes of ocean circulation, the biological input and the old carbon remineralization. These factors are affected by the changes of the sedimentary environment in the basin, and resulting in 14CTOC fluctuations. The element analyses by Itrax XRF core scanner and ICP-OES can reflect changes of sedimentary environment in the basin. The XRF results show elemental contents in the bulk sediments. The elemental concentrations of the acid-leachable fractions (ALE) measured by an ICP-OES represent elements mainly in the authigenic phases. Combining changes in the Δ14CTOC and the elemental contents in the core, the depositional history in SBB can be divided into three periods during the past ~200 years: (I) From 1870 to 1815 (37-51 cm depth): Lamination is not very identical. XRF scan shows high Si, K and Ti contents, but low Ca/Ti and Fe/Mn ratios, indicating mainly terrestrial sedimentary input. The ALE Ca and Sr concentrations were high but the other elements were low, reflecting fewer terrestrial elements in the marine authigenic mineral and adsorbed facies. The 14CTOC ages are much older than the depositional age due to old carbon influence. The old carbon comes from the POC and DOC input from terrestrial sources and remineralization of CH4 which is reduced from the old marine sediments in deeper layers. Part of the old carbons are oxidized to CO2 and mixed with DIC in seawater, elevating the reservoir age. The organic carbon generated in seawater utilizes the DIC, making the 14CTOC age older. The remaining terrestrial POC into the marine sediments will make the 14CTOC age even >2000-yr older than the depositional age. (II) From 1950 to 1870 (17-37 cm depth): Comparing with other periods, the terrigenous input become lower, and the concentration of ALE also decrease. During this period, the Δ14CTOC fluctuation became more negative, because of more oxidized environment, increasing oxygen into the sediments on the subsurface. Microorganism in the sediments promote the increase of remineralization of POC. The results of XRF show that the ratios of Fe/Mn and Ca/Ti all increase, and the contents of Si, K and Ti decrease, which shows that the terrestrial input decreases even though the sediments are still dominated by terrigenous sediments. Some calcareous shell formation, increased iron oxides and organic-rich lamina may indicate an increase in marine productivity during this time. The ALE contents decreased, indicating the reduction of river input. During this period, the 14CTOC became younger rapidly upward due to decreased old carbon influence. Increased vertical mixing of water column may be the main reason for enhancement of productivity and oxidation condition in SBB. (III) From 2012 to 1950 (0-17 cm depth): The lamination is obvious. Low Ca and the Fe/Mn ratio show enhanced reduction environment. The 14CTOC ages during this period have less reservoir age due to the input of nuclear bomb carbon. The lamination, XRF measured elements and ALEs as well as Δ14CTOC vary correspondingly probably to El Niño-Southern Oscillation (ENSO) effect. During the La Niña period, stronger upwelling and northerly current bring nutrient enriched water into SBB and lead to higher productivity. The organic and carbonate enriched sediments containing lower ALEs and lower XRF measured elements with higher Δ14CTOC form the light layer during La Niña period. During the El Nino period, the phenomena are opposite.