花東海盆濁流沉積的磁性特征及其環(huán)境意義

李波， 王艷， 鐘和賢， 張江勇， 李順， 李學(xué)杰， 高紅芳

1 國土資源部海底礦產(chǎn)資源重點實驗室， 廣州海洋地質(zhì)調(diào)查局， 廣州 510075 2 廣東省有色地質(zhì)勘查院， 廣州 510080

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花東海盆濁流沉積的磁性特征及其環(huán)境意義

李波1， 王艷2， 鐘和賢1， 張江勇1， 李順1， 李學(xué)杰1， 高紅芳1

1 國土資源部海底礦產(chǎn)資源重點實驗室， 廣州海洋地質(zhì)調(diào)查局， 廣州 510075 2 廣東省有色地質(zhì)勘查院， 廣州 510080

對取自臺灣以東花東海盆GX168孔的濁流沉積物進(jìn)行系統(tǒng)的巖石磁學(xué)研究，揭示其沉積學(xué)和巖石磁學(xué)特征，分析其物源和形成機(jī)制.研究結(jié)果顯示，剖面上共識別出12層濁流沉積物，其分布存在規(guī)律，下部350～700 cm共發(fā)育11層濁流沉積物，而0～350 cm僅出現(xiàn)1層濁流沉積物.濁流沉積物粒徑明顯較背景沉積物粗，石英、長石含量更高，底部與下伏背景沉積呈突變接觸，頂部與上覆背景沉積呈漸變接觸，內(nèi)部發(fā)育典型的正粒序韻律結(jié)構(gòu).濁流沉積物和背景沉積物具有相似的磁學(xué)特征，兩者均以磁鐵礦為主要載磁礦物類型，且磁鐵礦顆粒均以準(zhǔn)單疇和多疇顆粒為主.同時，兩者也存在一定差異，濁流沉積物中磁鐵礦較背景沉積物更為富集，磁化率和飽和等溫剩磁更強(qiáng)，磁鐵礦粒徑更粗，這與濁流沉積物原始沉積區(qū)更靠近物源區(qū)有關(guān).花東海盆濁流沉積形成的誘發(fā)機(jī)制可能是末次冰期以來頻發(fā)的海平面波動造成陸坡之上沉積物重力失穩(wěn)，導(dǎo)致陸坡沉積物向海盆搬運.

花東海盆； 濁流； 磁學(xué)特征； 海平面波動

1 引言

濁流(Turbidity current)是重力流的一種特殊形式，是一種在水體底部形成的高速湍流狀態(tài)的混濁流體，由水和大量懸浮泥砂混合組成(Kneller and Buckee, 2000).作為一種重要的地質(zhì)營力，濁流將大量淺海物質(zhì)搬運至深海再沉積，在淺海-深海的物質(zhì)交換與循環(huán)中發(fā)揮重要作用.濁流一方面為深海生態(tài)群帶來大量的陸源營養(yǎng)鹽，現(xiàn)場直接觀測證實濁流在維持深海峽谷繁盛的生態(tài)系統(tǒng)中發(fā)揮重要作用(徐景平, 2013)；另一方面會對沿途的底棲生物群落造成嚴(yán)重破壞，同時危及海底電纜等現(xiàn)代化設(shè)施(Hsu et al., 2008).而隨著油氣勘探的深入，人們發(fā)現(xiàn)濁流沉積體系同樣是一類重要的油氣儲層(龔建明等, 2005；Schneider et al., 2012).因此，濁流沉積受到國內(nèi)外學(xué)者廣泛關(guān)注，成為沉積學(xué)中的研究熱點(Mulder and Syvitski, 1995；Shanmugam, 1997, 2000；方愛民等, 1998；姜輝, 2010；Huang et al., 2012；高紅燦等, 2012；徐景平, 2014).

自然界中磁性礦物無處不在，其來源、搬運和沉積受氣候環(huán)境控制，利用巖石磁學(xué)手段識別沉積物中磁性礦物的類型、含量和粒徑，即可追蹤磁性顆粒的物質(zhì)來源、搬運與沉積等過程，從而反演氣候環(huán)境變化.經(jīng)過幾十年的發(fā)展，在海洋沉積物的環(huán)境磁學(xué)研究方面取得了諸多成果(Watkins and Maher, 2003；Kumar et al., 2005；孟慶勇和李安春, 2008；Peters et al., 2010；Liu et al., 2012；歐陽婷萍等, 2014).近20年來，國內(nèi)外報導(dǎo)了一些利用磁學(xué)手段研究濁流性質(zhì)及其環(huán)境意義的實例：李杰森等(1999)對云南撫仙湖現(xiàn)代濁流沉積物的磁化率研究發(fā)現(xiàn)，沉積物剖面中磁化率峰值與地震形成的濁流層具有良好的對應(yīng)關(guān)系，表明通過磁化率可揭示濁流的發(fā)育歷史；葛淑蘭等(2012)對日本海Ulleung盆地濁流沉積物的磁化率各向異性(AMS)研究表明，濁流沉積物樣品三軸主方向雜亂，證明AMS可作為沉積物剖面上濁流的識別手段之一；Anchuela等(2011)對始新世受構(gòu)造作用影響的濁積巖研究表明，鮑馬序列不同層位沉積巖的AMS方向相似.這些研究表明磁學(xué)手段在濁積物/巖的識別和沉積學(xué)特征研究上有效可行，但目前對濁流沉積詳細(xì)的環(huán)境磁學(xué)研究卻鮮見報道，濁流沉積物與原地背景沉積物的磁學(xué)性質(zhì)是否存在差異、有何差異尚不清楚.

西菲律賓海陸架狹小，四周被海溝包圍，陸坡陡，具備形成濁流的潛力，近年來的海洋地質(zhì)調(diào)查與研究工作證實了西菲律賓海及鄰區(qū)沉積物中發(fā)育濁流沉積(李軍等, 2005；黨皓文等, 2009；仇曉華等, 2012).因此，本文選取臺灣島東側(cè)花東海盆GX168孔為研究對象，通過詳細(xì)的巖石磁學(xué)分析，弄清濁流沉積物和背景沉積物各自的磁學(xué)性質(zhì)、兩者之間的差異及環(huán)境意義，并結(jié)合區(qū)域地質(zhì)背景分析濁流事件的成因.

2 樣品采集和實驗方法

GX168孔重力活塞樣取自臺灣島東側(cè)花東海盆西南角，其位于呂宋火山島弧東側(cè)，加瓜海脊西側(cè)(圖1)，水深4409 m，巖心長700 cm.柱狀樣巖心沿中軸線剖開后，用邊長2 cm的無磁性立方體樣品盒對其中一半巖心取樣，間距為10 cm，共采集磁學(xué)樣品71個.

樣品稱重后依次進(jìn)行如下環(huán)境磁學(xué)項目測量：(1)利用卡帕橋MFK1-FA多頻磁化率儀測量所有樣品的低頻(976 Hz)和高頻(15616 HZ)磁化率，記為LF和HF，然后計算頻率磁化率百分?jǐn)?shù)fd%=(LF-HF)/LF×100%；(2)樣品在100 mT交變場疊加0.05 mT的直流偏置場下獲得非磁滯剩磁(ARM)；(3)利用2G Enterprise 脈沖磁化儀獲得外場為1 T條件下的等溫剩磁(將之視為飽和等溫剩磁(SIRM))，之后將樣品在300 mT反向場中磁化，得到IRM-300mT，定義S-ratio=-IRM-300 mT/SIRM，HIRM=(SIRM+IRM-300 mT)/2(Thompson and Oldfield, 1986；Evans and Heller, 2003)；(4)上述所有剩磁測量均在2G-760 U-Channel 巖石超導(dǎo)磁力儀上完成，靈敏度為2×10-12Am2；(5)利用卡帕橋MFK1-FA多頻磁化率儀及CS-3溫控裝置測量代表性樣品的磁化率隨溫度變化曲線(κ-T曲線)：粉末樣品在氬氣環(huán)境下自室溫以11 ℃·min-1速率升溫至700 ℃后，以相同速率冷卻至室溫，測量頻率為976 Hz，磁場強(qiáng)度為200 A·m-1；(6)在Micromag 3900振動磁力儀上完成代表性樣品的磁滯參數(shù)(飽和磁化強(qiáng)度Ms，飽和剩余磁化強(qiáng)度Mrs，矯頑力Bc和剩磁矯頑力Bcr)以及IRM獲得曲線測量(最大場設(shè)定為1 T，步長為5 mT).所有磁學(xué)實驗均在中國科學(xué)院地質(zhì)與地質(zhì)物理研究所古地磁與年代學(xué)實驗室完成.

圖1 GX168孔位置和西菲律賓海海底地形(a) 底圖改繪自Smith和 Sandwell (1997); (b) 水深數(shù)據(jù)引自SRTM30_PLUS V6.0 (Becker et al., 2009).Fig.1 Location of core GX168 and submarine relief of the West Philippine Sea(a) The base map was modified from Smith and Sandwell (1997); (b) The bathymetric data was cited from SRTM30_PLUS V6.0 (Becker et al., 2009).

3 結(jié)果與分析

3.1 濁流沉積發(fā)育特征

由于濁流與碎屑流的相似性，導(dǎo)致實際研究中一部分碎屑流或底流被誤認(rèn)為是濁流沉積(高紅燦等, 2012)，因此對濁流沉積的正確鑒別至關(guān)重要.濁流沉積與碎屑流沉積外在特征最顯著的區(qū)別是濁流沉積具正粒序韻律結(jié)構(gòu)，底部為突變接觸而頂部為漸變接觸(Shanmugam, 1997, 2000)；而碎屑流沉積一般具上、下兩層韻律結(jié)構(gòu)，即下部發(fā)育具平行碎屑結(jié)構(gòu)的層流段，上部發(fā)育具有塊狀層理的“剛性”筏流段，頂?shù)拙鶠橥蛔兘佑|(王德坪，1991；Bridge and Demicco, 2008).

研究區(qū)位于深海盆地，水深超過4000 m，廣義而言，靜水泥質(zhì)沉積可作為背景沉積(正常沉積)，其他類型沉積均可視為事件沉積(非正常沉積)，它們具有特殊的形成機(jī)制與沉積模式.參照張富元等(2012)的分類命名方案，GX168孔沉積物中背景沉積主要是灰色細(xì)粒遠(yuǎn)洋粘土，以粉砂和粘土粒級顆粒為主，砂含量極少(通常小于1%)，鏡下未見鈣質(zhì)生物殼體，為典型的深海沉積.在這些深海沉積中共識別出12層事件沉積(僅在其中6層采集了磁學(xué)樣品)，其在剖面上的分布存在一定規(guī)律(圖2a)：0～350 cm事件沉積發(fā)育明顯較少，僅見1層事件沉積；350～700 cm事件沉積發(fā)育頻率明顯增大，共識別11層事件沉積.事件沉積單層厚度2～9 cm，其底部沖刷侵蝕下伏背景沉積，兩者呈突變接觸，接觸面清晰截然.事件沉積內(nèi)部發(fā)育正粒序韻律結(jié)構(gòu)，頂部與上覆背景沉積呈漸變接觸，接觸面不明顯(圖2b).以上特征可確定這些事件沉積為濁流成因.在物質(zhì)組成上，濁流沉積物顏色相對背景沉積物稍淺，顆粒粒徑明顯更粗，以砂為主，砂感強(qiáng)，粉砂、粘土顆粒含量明顯降低，定名為粉砂質(zhì)砂-砂(Folk et al., 1970).在濁流內(nèi)部，從底到頂，顆粒粒徑明顯減小，由砂漸變成粉砂質(zhì)砂(即正粒序結(jié)構(gòu)，圖2).濁流沉積物在鏡下可見無色—白色粗粒石英、長石顆粒含量明顯較上下層位(背景沉積)增多，這正是導(dǎo)致濁流沉積物顆粒相對背景沉積物顏色變淺、顆粒變粗的原因，這一特征與南海南部巽他陸坡底部MD05-2895孔的濁流沉積物相似(趙玉龍等, 2011).此外，部分濁流層內(nèi)可見少量—較豐富有孔蟲殼體，這些特征均反映濁積物來自相對淺水的海底沉積，并非原地深海沉積.

3.2 磁性礦物的類型

圖3 GX168孔典型沉積物樣品的IRM獲得曲線及反向場退磁曲線是樣品在正向磁場中獲得剩磁為SIRM值一半時的磁場，星號標(biāo)記的樣品取自濁流層(下同)，括號內(nèi)標(biāo)注了樣品的深度(下同)，結(jié)果顯示濁流沉積物和背景沉積物無明顯差別.Fig.3 Isothermal remanent magnetization (IRM) acquisition curves and backfield acquisition curves of typical samples of core GX168 sediments is defined as the magnetic field strength required to magnetize a sample to one-half of its saturation isothermal remanent magnetization. Sample marked with an asterisk was taken from turbidite layer (similarly hereinafter). The depth of the sample is shown within the parentheses (similarly hereinafter). The results indicate that there are no significant differences between the turbidite and background sedimentation.

不同的磁性礦物在加熱/冷卻過程中，其磁化率值會隨溫度變化(κ-T曲線)而表現(xiàn)不同特征，根據(jù)這些特征可判斷樣品中磁性礦物的類型及礦物相之間的轉(zhuǎn)變(Hrouda, 1994；Dunlop and ?zdemir, 1997；Liu et al., 2005).GX168孔典型沉積物樣品的κ-T曲線如圖4所示，可分為兩類.第一類樣品(圖4a)加熱曲線的磁化率一直上升，在300 ℃左右時達(dá)到峰值，之后隨溫度升高而迅速下降，但在500 ℃之后再次上升，并在560 ℃左右形成第二個峰值(可能是細(xì)粒單疇(SD)顆粒磁鐵礦解阻形成的Hopkinson峰)，之后隨溫度升高而迅速下降，并在磁鐵礦的居里溫度點(580 ℃)趨于零，冷卻曲線顯示其在580 ℃左右磁化率開始增加，其終止磁化率同樣比初始磁化率高很多，表明在加熱過程中有新的磁鐵礦形成.第二類樣品(圖4b—4f)加熱曲線的磁化率一直上升，并在350 ℃左右達(dá)到峰值，之后隨溫度升高而迅速下降，并在580 ℃趨于零，其冷卻曲線顯示在580 ℃左右磁化率開始增加，其終止磁化率比初始磁化率高很多，表明在加熱過程中有新的磁鐵礦形成.所有樣品加熱和冷卻過程均顯示不可逆特征，冷卻后的磁化率值明顯高于初始值，指示樣品在加熱過程中生成了新的亞鐵磁性礦物，這些新生成的磁鐵礦可能來自于樣品中含鐵硫化物、含鐵硅酸鹽或粘土礦物的受熱轉(zhuǎn)換(Ellwood et al., 2007；楊小強(qiáng)等, 2007；歐陽婷萍等, 2014).

磁滯回線形態(tài)和閉合特征可用來判斷磁性顆粒類型.圖5是GX168孔濁流沉積物與背景沉積物代表性樣品經(jīng)過順磁校正后的磁滯回線.總體而言，全部樣品的磁滯回線形態(tài)非常類似，差別很小.所有樣品磁滯回線的腰均很細(xì)，即Bc較小，在100 mT外加磁場下磁滯回線已經(jīng)完全閉合，表明樣品中載磁礦物主要是亞鐵磁性礦物，基本不含硬磁性礦物(Dunlop and ?zdemir, 1997).

3.3 磁性礦物的粒徑

傳統(tǒng)磁學(xué)理論認(rèn)為，磁疇處于SP/SD閾值區(qū)域的顆粒，其磁化率值與弛豫時間有關(guān)，弛豫時間越短，顆粒振動越快.當(dāng)觀測的時間尺度小于顆粒振動周期時，則觀測比顆粒振動快，顆粒顯示穩(wěn)定單疇的信息，磁化率值低；反之，當(dāng)觀測的時間尺度大于其顆粒振動周期，則觀測比顆粒振動慢，顆粒表現(xiàn)出超順磁特征，磁化率值高.因此，不同頻率下測定的磁化率差值可用來表征磁疇位于SP/SD閾值區(qū)域內(nèi)磁性顆粒的含量(劉青松和鄧成龍, 2009；徐新文等, 2012).GX168孔沉積物樣品的fd%十分穩(wěn)定，分布范圍為1.55～3.28，平均值2.18，濁流沉積物和背景沉積物差別不大，表明GX168孔所有沉積物樣品中處于SP/SD閾值區(qū)域的顆粒含量均很低.

圖4 GX168 孔典型沉積物樣品的κ -T曲線樣品在氬氣環(huán)境下以11 ℃·min-1速率加熱到700 ℃后以相同速率冷卻至室溫，所有樣品的κ -T曲線可分為兩類：a為一類，b-f為一類.Fig.4 κ -T curves of typical samples of core GX168 sediments All samples were heated from room temperature to 700 ℃ at a heating rate of 11 ℃·min-1 and then cooled at the same rate. κ -T curve of all samples could be divided into two categories: sample (a) and the remanent five samples (b—f).

圖5 GX168孔典型沉積物樣品的磁滯回線Fig.5 Magnetic hysteresis loops of typical samples of core GX168 sediments

圖6 GX168孔沉積物樣品的King圖Fig.6 King plot of core GX168 sediments

圖7 GX168孔典型沉積物樣品的Day圖Fig.7 Day plot of typical samples of core GX168 sediments

當(dāng)樣品中主要載磁礦物為磁鐵礦時，可用King圖(King et al., 1982)或Day圖(Day et al., 1977)來確定磁性礦物的顆粒大小.GX168孔沉積物樣品的King圖(圖6)投影表明大部分樣品中磁性顆粒的粒徑分布較集中，粒徑均大于1 μm，濁流沉積物樣品和背景沉積物樣品分布在兩個明顯不同的區(qū)域，濁流沉積物樣品的粒徑明顯更粗一些，均大于5 μm.將經(jīng)過順磁校正的樣品磁滯參數(shù)投影到Dunlop(2002)修訂過的Day圖中發(fā)現(xiàn)，濁流樣品和背景沉積物樣品的分布相似，兩者各有一部分樣品落在PSD及MD區(qū)域，但可看出濁流樣品的顆粒明顯更粗(圖7)，與King 圖投影結(jié)果一致.

3.4 磁性參數(shù)剖面變化

S-ratio可很好地反映樣品中軟磁組分和硬磁組分的相對含量.S-ratio越接近1，載磁礦物中軟磁組分的相對含量越高；反之，隨著S-ratio降低，硬磁組分的相對含量增多(Thompson and Oldfield, 1986).GX168孔沉積物樣品的S-ratio普遍介于0.96～1.00之間，平均值0.98，表明樣品中載磁礦物以低矯頑力軟磁性組分(亞鐵磁性礦物)為主；而且S-ratio隨深度增加變化較小(圖8)，濁流沉積物和背景沉積物之間并無明顯差異，指示整個剖面上載磁礦物組分十分穩(wěn)定.

HIRM主要與硬磁礦物(如赤鐵礦、針鐵礦等)的含量有關(guān)，硬磁組分的絕對含量越高，HIRM值越大(Thompson and Oldfield, 1986).GX168孔沉積物樣品的HIRM最小值為4.22×10-7Am2·kg-1，最大值為5.43 ×10-4Am2·kg-1，平均值為 1.80×10-4Am2·kg-1，表明赤鐵礦類硬磁性礦物的含量整體較低.從剖面上看，HIRM變化規(guī)律不明顯，與其他磁性參數(shù)，特別是與的相關(guān)性較差，表明的變化與硬磁組分含量關(guān)系不大.而濁流沉積物樣品和背景沉積物樣品的HIRM并未表現(xiàn)出明顯差異，表明兩者中赤鐵礦等硬磁礦物的絕對含量均較低，差別不大，與S-ratio揭示的規(guī)律一致.

圖8 GX168孔磁性參數(shù)剖面變化圖陰影部分標(biāo)示濁流發(fā)育層位，共12層；星號表示樣品取自于濁流層(共6層，采樣深度分別是370 cm、560 cm、570 cm、600 cm、620 cm與690 cm)，未標(biāo)星號的濁流層未取樣；5×10-6 m3·kg-1為濁流沉積物和背景沉積物樣品的磁化率值分界線.Fig.8 Stratigraphic variation of magnetic parameters of core GX168Twelve turbidite layers are marked by shading. The asterisk indicates that the samples were taken from the turbidite layers (the depths of samples are 370 cm, 560 cm, 570 cm, 600 cm, 620 cm and 690 cm, respectively). No sample was collected from the turbidite layer without an asterisk marked. 5×10-6 m3·kg-1 is the boundary of magnetic susceptibility between the turbidite and background sedimentation.

圖9 GX168孔沉積物的SIRM與 相關(guān)性關(guān)系圖Fig.9 Linear correlation between SIRM and of core GX168 sediments

4 討論

4.1 濁流沉積與背景沉積磁性特征差異及原因

利用磁學(xué)性質(zhì)分析沉積物蘊(yùn)含的環(huán)境意義，首先必須弄清沉積物中磁性礦物的成因，因為碎屑成因的磁性礦物才具有物源信息，而生物成因磁性礦物形成于早期成巖過程中，不代表原始磁學(xué)特征.Oldfield(1994)對湖泊和海洋近岸沉積物中細(xì)粒亞鐵磁性礦物的成因深入研究后，提出采用ARM/fd和ARM/兩個參數(shù)組合判別磁鐵礦成因：如果ARM/和ARM/fd均很大，尤其是后者大于1000時，樣品中的磁鐵礦主要由細(xì)菌生成.GX168孔包括濁流沉積物在內(nèi)所有樣品的ARM/介于0.57～2.32之間，平均值1.29，ARM/fd介于29.38～82.88之間，平均值58.36，兩者均很低，尤其是后者遠(yuǎn)低于判別標(biāo)準(zhǔn).因此，可認(rèn)為GX168孔中磁性礦物為碎屑成因.而普遍較低的SIRM/(圖8)表明鉆孔沉積物中不含早期還原成巖作用形成的亞鐵磁性硫化物(Peters and Dekkers, 2003)，說明早期成巖作用對本鉆孔沉積物的影響較小，基本保存了原始沉積信息.

濁流沉積物和背景沉積物在磁性礦物含量和粒徑上存在顯著差異(表1)：表征磁性礦物含量的參數(shù)、SIRM和ARM顯示濁流沉積物中磁性礦物含量明顯較背景沉積物高，與撫仙湖現(xiàn)代濁流沉積物的特征相同(李杰森等, 1999)；ARM/值、King圖及Day圖投影表明濁流沉積物中磁鐵礦粒徑明顯大于背景沉積物.陸源物質(zhì)通過河流搬運到海洋，然后在底流作用下分散到盆地中沉積.濁流沉積物原始沉積在相對更淺的陸架區(qū)，離陸源區(qū)更近，單位時間內(nèi)“捕獲”到的陸源物質(zhì)通量更大(導(dǎo)致沉積物磁性增強(qiáng))，粒度也更粗；而背景沉積區(qū)因更遠(yuǎn)的搬運距離導(dǎo)致輸入到沉積區(qū)磁性礦物的通量降低，粒徑變細(xì).濁流沉積物和背景沉積物具有相同的物源，因而磁性礦物的種類相同，而離物源區(qū)不同的距離導(dǎo)致了兩者磁性礦物含量和粒徑上的差異.在利用巖石磁學(xué)參數(shù)解釋氣候環(huán)境時必須注意沉積序列中濁流沉積層，否則可能會對剖面上的磁化率“異常高值”做出錯誤的解釋.

表1 GX168孔濁流沉積與背景沉積磁學(xué)參數(shù)對比Table 1 The magnetism parameters of turbidite compared with background sedimentation of core GX168

4.2 濁流發(fā)育成因

濁流沉積是大陸邊緣沉積物向深海區(qū)的搬運，必須滿足一定條件才能形成.廣義上講，沉積重力流需滿足四個條件：足夠的水深、充足的物質(zhì)、必要的坡度和觸發(fā)機(jī)制.前兩點是濁流發(fā)育的物質(zhì)基礎(chǔ)，第三點是維持濁流運動的動力來源.對海洋環(huán)境而言，濁流主要形成于陸棚坡折以下的相對深水區(qū)，特別是盆地中的三角洲前緣、洋盆中的海底峽谷口和平行于克拉通邊緣的深海槽等環(huán)境(姜輝, 2010).最后一個條件尤為關(guān)鍵，是突發(fā)性外力對沉積物靜力學(xué)不穩(wěn)定破壞.前人研究表明，深海濁流形成的觸發(fā)機(jī)制主要有海平面波動、地震、火山作用、海嘯巨浪和風(fēng)暴潮、天然氣水合物泄露等，處于較陡地形之上的松散沉積物在上述這些事件發(fā)生時重力失穩(wěn)，極易發(fā)生垮塌，順著陸坡向深海區(qū)搬運，形成濁流沉積(Lee et al., 1996；Rothwell et al., 1998；方愛民等, 1998；黨皓文等, 2009；姜輝, 2010；仇曉華等, 2012；Arai et al., 2013).GX168孔所在區(qū)域水深4409m，位于花東海盆西側(cè)斜坡底部，距離呂宋火山島弧不足30 km，距離臺灣島200 km，物質(zhì)來源充足，其西側(cè)的陸坡坡度很陡(圖1)，為濁流發(fā)育提供了極為有利的地形和必要的物質(zhì)基礎(chǔ)，只要外力觸發(fā)，很容易形成濁流.

研究區(qū)位于歐亞板塊與菲律賓海板塊的碰撞地帶，頻發(fā)的火山活動似乎可觸發(fā)濁流的形成.但GX168孔濁流沉積發(fā)育的臨近層位并未發(fā)現(xiàn)火山灰堆積層，鏡下觀察也少見火山物質(zhì)的蹤影，因而構(gòu)造運動導(dǎo)致的火山活動似乎與本區(qū)濁流形成的關(guān)系不大.對取自沖繩海槽南部A23孔中濁積層的研究表明，濁流可能主要由地震觸發(fā)形成(李軍等, 2005).花東海盆位于板塊會聚、地震活動區(qū)，構(gòu)造運動引起的頻發(fā)地震可能是濁流形成的觸發(fā)機(jī)制，但從濁流在剖面上的分布規(guī)律來看，350 cm以下濁流發(fā)育的頻率明顯要高于上部(0～350 cm)，表明全新世以來濁流發(fā)育的頻率明顯變緩，上部沉積物未能響應(yīng)近幾十年來頻發(fā)的地震(方興義, 2014)，因而地震觸發(fā)機(jī)制還需更多資料的支持.對西菲律賓海呂宋島岸外柱狀樣中濁流沉積研究表明，末次冰期以來海平面劇烈波動(Hanebuth et al., 2000)造成欠固結(jié)沉積物重力失穩(wěn)是促發(fā)濁流形成的主要原因，低水位時期陸源物質(zhì)堆積是必要條件，而海平面上升后造成的沉積物不穩(wěn)定是觸發(fā)因素之一(黨皓文等, 2009；仇曉華等, 2012).趙玉龍等(2011)對南海南部巽他陸坡底部濁流沉積研究后，同樣認(rèn)為海平面波動是造成巽他陸坡上濁流形成的原因.本文由于缺乏有孔蟲氧同位素曲線和放射性同位素年齡資料，因而無法詳細(xì)探討本區(qū)濁流沉積與海平面波動之間的聯(lián)系，但GX168孔濁流發(fā)育的頻率特征與菲律賓海西部呂宋島岸MD98-2188孔揭示的濁流發(fā)育規(guī)律非常相似(黨皓文等, 2009)，均是沉積物剖面下部濁流發(fā)育頻繁，而上部濁流發(fā)育頻率明顯降低.因此，海平面變化導(dǎo)致本區(qū)濁流發(fā)育的可能性最大.末次冰期以來，全球海平面波動十分頻繁，發(fā)生了多期海平面快速升降事件，末次盛冰期時海平面最大下降甚至可達(dá)百米(Fairbanks, 1989; Hanebuth et al., 2000; Siddall et al., 2003)，導(dǎo)致西菲律賓海及臺灣島東側(cè)陸坡地區(qū)濁流沉積頻繁發(fā)育，但進(jìn)入全新世后，全球海平面逐漸趨于穩(wěn)定，濁流發(fā)育的頻率隨之減緩.

5 結(jié)論

本文對采自臺灣島東側(cè)花東海盆南部的GX168孔濁流沉積物進(jìn)行詳細(xì)的巖石磁學(xué)分析，主要得出以下結(jié)論：

(1) GX168孔發(fā)育12層粉砂質(zhì)砂—砂層，以砂為主，粉砂、粘土相對較少，具典型的正粒序韻律結(jié)構(gòu)，沉積物粒徑明顯較背景沉積物(粘土)粗，底部與下伏沉積呈突變接觸，頂部與上覆沉積呈漸變接觸，鑒定為濁流沉積.鏡下觀察表明濁流沉積物相對背景沉積物顏色變淺、粒徑增大主要與粗粒石英、長石含量相對富集有關(guān).

(2) 詳細(xì)的巖石磁學(xué)分析表明，濁流沉積物和背景沉積物具有相似的磁學(xué)特征，表現(xiàn)在兩者均以磁鐵礦為主要載磁礦物類型，磁鐵礦粒徑均以PSD和MD顆粒為主.但兩者也存在差異：濁流沉積物中磁鐵礦的含量明顯較背景沉積物富集，磁化率和飽和等溫剩磁更強(qiáng)，磁性礦物粒徑也更粗，與濁流原始沉積區(qū)更靠近物源有關(guān).兩者較大的磁學(xué)性質(zhì)差異在進(jìn)行氣候環(huán)境解釋時必須引起重視.

(3) 花東海盆濁流沉積形成的誘發(fā)機(jī)制可能是末次冰期以來頻發(fā)的海平面波動，海平面波動造成陸坡之上松散沉積物重力失穩(wěn)，導(dǎo)致陸坡區(qū)沉積物向海盆搬運.

致謝 巖石磁學(xué)測試得到了中國科學(xué)院地質(zhì)與地球物理研究所古地磁與年代學(xué)實驗室劉青松和周燦芬老師的協(xié)助，審稿專家提出了寶貴意見，在此一并感謝！

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(本文編輯 胡素芳)

Magnetic properties of turbidites in the Huatung Basin and their environmental implications

LI Bo1, WANG Yan2, ZHONG He-Xian1, ZHANG Jiang-Yong1, LI Shun1, LI Xue-Jie1, GAO Hong-Fang1

1KeyLaboratoryofMarineMineralResources,GuangzhouMarineGeologicalSurvey,MinistryofLandandResources,Guangzhou510075,China2GuangdongNonferrousMetalsGeologicalExplorationInstitution,Guangzhou510080,China

Turbidity currents are part of a continuum of sediment-gravity flows, and are traditionally defined as those sediment-gravity flows in which sediment is suspended by fluid turbulence. Turbidity currents are principal mode of transportation of clastic grains to the deep ocean. The occurrence of turbidity current plays an important role in the formations of canyon, submarine fan and deep-sea oil. Previous study shows lots of turbidite records were identified in the West Philippine Sea and adjacent regions. However, few works focus on the magnetic properties of the turbidites in this area. In this paper, systematic rock-magnetism measurements, including magnetic susceptibility, anhysteretic remanent magnetization, isothermal remanent magnetization, saturation isothermal remanent magnetization, thermomagnetism and hysteresis loops were performed on the marine sediments of core GX168 from the Huatung Basin to uncover the sedimentological characteristics and magnetic features of turbidite sequences and the differences with the background sedimentation, also, further explore the provenance and formation mechanism.

Huatung Basin； Turbidity current； Magnetic properties； Sea-level fluctuation

10.6038/cjg20160917.

國土資源部海底礦產(chǎn)資源重點實驗室開放基金課題(KLMMR-2014-A-12)，國家自然科學(xué)基金項目(41306042,41272384)，中國地質(zhì)調(diào)查局項目(GZH201300502,GZH201500202,DD20160227)資助.

李波，男，1986年生，博士，工程師，主要從事海洋區(qū)域地質(zhì)調(diào)查和研究工作.E-mail：libo_cug@163.com

10.6038/cjg20160917

P736

2016-04-28，2016-06-16收修定稿

李波， 王艷， 鐘和賢等. 2016. 花東海盆濁流沉積的磁性特征及其環(huán)境意義. 地球物理學(xué)報，59(9):3330-3342,

Li B, Wang Y, Zhong H X, et al. 2016. Magnetic properties of turbidites in the Huatung Basin and their environmental implications.ChineseJ.Geophys. (in Chinese),59(9):3330-3342,doi:10.6038/cjg20160917.

There are overall 12 turbidite layers in the core GX168. There was significant regularity in the distribution of turbidite on profile: 11 turbidite layers distributed in the interval from 350 cm to 700 cm in depth, but only one turbidite layer was found from the interval of 0～350 cm in depth. The results show turbidite characterized by sand and silty sand in lithology contain more coarse sediment grains than background sedimentation with dominant lithology of clay. The content of quartz and feldspar in turbidite is higher than that of background sedimentation, which is the reason of lighter colors of turbidite than that of background sedimentation. The contact between bottom boundary of turbidite succession and underlying sediments is generally sharp and instant; while the contact between the top boundary of turbidite and overlying sediments is transitional transition. The turbidite is characterized by typical graded bedding sequence.

Turbidite and background sedimentation share similar magnetic properties including: magnetites are the dominant magnetic carries for both of them, and the grain sizes of magnetite particles are both pseudo-single-domain (PSD) and multi-domain (MD). However, there are some differences between them: the turbidite enrich more magnetic minerals than background sedimentation obviously; the magnetic susceptibility and saturation isothermal remanent magnetization of turbidite is stronger in comparison to that of background sedimentation, and the grain size of magnetite in turbidite is coarser than that of background sedimentation. The similar magnetic properties between the turbidite and background sedimentation suggest the same provenance. However, the differences of concentration and grain size between them were resulted by primary depositional setting of turbidite closer to the provenance than background sedimentation. The occurrence of these turbidity currents sedimentation in the Huatung basin is implied to be potentially related to the gravity-induced slumping of sediments on slope (e.g. slope failure) that caused by sea-level fluctuation frequently during the Last Glaciation, and the sediments on the slope were transported to the deep basin.