Geochemistry study on a sediment core from Santa Barbara Basin, California, USA

• Main Authors: Yi-Wei Chang, 張益瑋
• Other Authors: Hong Chun Li
• Format: Others
• Language: zh-TW
• Published: 2018
• Online Access: http://ndltd.ncl.edu.tw/handle/v2wnx4

碩士 === 國立臺灣大學 === 地質科學研究所 === 106 === 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.