西北太平洋ODP 1210站位上中新統至下更新統鈣質超微化石生物地層研究與應用

Upper Miocene-lower Pleistocene calcareous nannofossil biostratigraphy of ODP 1210, northwestern Pacific and some application

林荷雅(Ho-Ya Lin)

指導教授 ： 魏國彥

國立臺灣大學/理學院/地質科學研究所/碩士(2012年)

https://doi.org/10.6342/NTU.2012.01788

摘要

鈣質超微化石是地層學研究中極佳的年代對比工具。生物地層定年利用地層中的生物事件來界定出生物帶；生物事件包括生物的始現面(First Occurrence, FO or First Appearance Datum, FAD)及末現面(Last Occurrence, LO or Last Appearance Datum, LAD)等。我們運用鈣質超微化石生物定年時，傾向將這些生物事件視為等時性(synchronous)事件。然而，全球海洋面積廣大，各海域間的溫鹽條件、地理環境皆有差異，使得生物在各大洋間的演化情況不一，可能造成生物事件呈現「異時性」(diachronous)的問題。近年，隨著海洋鑽探研究資料的累積，再加上古地磁地層研究的發展，逐漸能分辨出各個生物事件在不同地區的「異時性」(diachroniety)。本研究分析西北太平洋ODP 1210岩芯晚中新世至早更新世的生物地層，辨認出十一個重要的生物事件，經與其他地區對比，對於數個鈣質超微化石生物帶的「異時性」現象有所評估。
ODP 1210岩芯 (32°13N, 158°15E，水深2573.6公尺，岩芯總長241.4公尺)位於日本東側海域的海底高原Shatsky Rise。本研究自岩芯深度32.38-95.36公尺，每間隔50公分採樣，作相對豐度分析。接近於生物界面則加密為間隔10公分取樣，並利用ODP 198航次報告中所提供的古地磁資料作為年代上的控制點，內插推估出各生物事件時間，建立本區域上新世的鈣質超微化石定年架構。研究結果顯示ODP 1210晚中新世至早更新世的沉積物樣本中可以辨識出十一個鈣質超微化石生物事件，年代由新到老依序如下：
(1) LCO of Discoaster brouweri (1.96 Ma)
(2) LO of Discoaster pentaradiatus (2.51 Ma)
(3) LO of Discoaster surculus (2.52 Ma)
(4) LO of Discoaster asymmetricus (2.81 Ma)
(5) LO of Discoaste tamalis (2.85 Ma)
(6) LCO of Reticulofenestra pseudoumbilica (3.82 Ma)
(7) FO of Discoaster tamalis (3.98Ma)
(8) LO of Amaurolithus spp. (4.00Ma)
(9) FO of Discoaster asymmetricus (4.12 Ma)
(10) FO of Ceratolithus spp. (5.00 Ma)
(11) LO of Discoaster quinqueramus(5.86 Ma)
其中，Discoaster brouweri LCO、Reticulofenestra pseudoumbilica LCO、Discoaster asymmetricus FO為絕佳的全球等時性事件。Discoaster surculus LO、Discoaster tamalis LO、Discoaster quinqueramus LO為異時性事件。
P. lacunose FO的年代為3.84 Ma，位在R. pseudoumbilica LO界面之下，P. lacunosa與R. pseudoumbilica (>7 μm)有短暫的共存期。R. pseudoumbilica的冷水型亞種R. gelida 在本研究中常見，LCO R. gelida與R. pseudoumbilica LCO同時發生，推論在西北太平洋水域中，控制R. gelida與R. pseudoumbilica絕滅的因子是相同的。R. pseudoumbilica LO為絕佳的全球等時性事件，但作為生物界面的指標化石，必須以形態大小超過7 μm作為標準，才具有等時性。本研究未有暖水種Sphenolithus abies連續出現，可能暗示Shatsky Rise在3.5 Ma時的海水條件，不適合S. abies生存。
ODP 1210的Amaurolithus與Ceratolithus兩個屬的豐度很低，僅能以Amaurolithus delicatus LO取代Amaurolithus tricorniculatus LO，作為NN14的上界事件。Ceratolithus rugosus FO在西北太平洋的生物地層應用上，可能不適用，僅能判定出Ceratolithus spp. FO。本文因此以Ceratolithus spp. FO作為NN12之上界。
運用ODP 1210的超微化石結果檢討出磺坑剖面的鈣質超微化石分帶的層界，依據早期的前人研究的資料作討論，有以下結論︰NN13/NN14界面在十六份頁岩中段，NN12/NN13實際界面無法確定。NN14/NN15的界面可以認定在魚藤坪砂岩段底部。利用P. lacunosa FO，認為NN15/NN16應修訂在錦水頁岩/魚藤坪砂岩之交界處。Matuyama/Gauss古地磁界面位於卓蘭層底部，綜合磁地層與超為化石資料，作者認為NN17的上界與下界皆在卓蘭層之中。

關鍵字

鈣質超微化石生物定年 ； 上新統 ； 異時性 ； Shatsky Rise ； 出磺坑剖面

並列摘要

The conventional nannofossil standard zonations of Martini (1971) and Okada & Bukry (1980) have been prime standards for biostratigraphic correlation of Neogene sediments. Advance in biochronology in the last four decades has further calibrated the first and last occurrences of index species in those schemes. However, these two zonations were mainly based upon low-latitudes stratigraphic sections of marine sediments. The diachroniety of each bioevent needs to be evaluated when such standard schemes are employed in middle latitudes. Particularly, the first and last occurrences of warm-water Discoaster spp. tend to be diachronous between low- and mid-latitudes.
This study analyzed the upper Miocene-lower Pleistocene calcareous nannofossil assemblages from a mid-latitude ODP site in the northern Pacific to address the fore-mentioned issues. The studied site is ODP 1210 (32°13N, 158°15E; water depth of 2574 m) which located on the Shatsky Rise.
This study examined the samples at 50 cm intervals with semi-quantitative abundance of nannofossils, and prepared all slides with processed standard techniques.All slides were analyzed under the light microscope at ×1600 magnification. Moreover, the bioevents were determined by higher resolution which was 10 cm sampling interval and counting the key fossil number within a total as 500 specimens.
The age of 11 nannofossil datums is estimated by interpolation between magnetic reversals. They are: LCO of Discoaster brouweri at 1.96 Ma, LO of D. pentaradiatus at 2.51 Ma, LO of D. surculus at 2.52 Ma, LO of D. asymmetricus at 2.81 Ma, LO of D. tamalis at 2.85 Ma, LCO of Reticulofenestra pseudoumbilica at 3.82 Ma, FO of D. tamalis at 3.98 Ma, LO of Amaurolithus spp. at 4.00 Ma, FO of D. asymmetricus at 4.12 Ma, FO of Ceratolithus spp. at 5.00 Ma, LO of D. quinqueramus at 5.86 Ma. Three bioevents are evaluated to be diachronous, including LO of D. surculus, LO of D. tamalis, and LO of D. quinqueramus.
3.84 Ma for the FO of Pseudoemiliania lacunosa recorded slightly below the LCO of R. pseudoumbilica. Amaurolithus spp.and Ceratolithus spp. are presented in rare numbers, hence the LO of A. tricorniculatus and the FO of C. rugosus are unavailable in ODP 1210. Especially, that the LCOs of R. gelida and R. pseudoumbilica at 3.82 Ma are identical suggested the fates of both morphotypes were controlled by the same factor. Reticulofenestra gelida may represented a winter morphovariant of R. pseudoumbilica. The absence of a warm-water species Sphenolithus abies implies that the water masses throughout the late Miocene-early Pliocene at this location were too cold for Sphenolithus abies to live, but still warm enough for various Discoaster species.

並列關鍵字

nannofossil biochronology ； Pliocene ； diachroniety ； Discoaster ； Shatsky Rise

參考文獻

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