Rates of Holocene Relative Sea-Level Rise and Differential Crustal Movements in the Mississippi Delta

全球变暖导致的海平面加速上升预计将成为密西西比三角洲的一个巨大问题，该地区沿海湿地正以每年约100平方公里的速度急剧减少。为了促进未来的湿地管理策略，正确理解海平面上升的自然速率及其由不同构造沉降速率引起的空间变异性是必不可少的。最近对其他主要三角洲平原的研究表明，空间上不均匀的地壳运动可能相当大。目前可获得的密西西比三角洲的海平面数据并不能解决不同的构造沉降率，而墨西哥湾沿岸的大多数全新世相对海平面曲线显示出一个明显的，但通常相互冲突的“阶梯”模式，即海平面静止和快速上升交替。这些现象的尺度比其他地方推断的与气候有关的海平面细微波动（1米）要大一个数量级，而且它们很难与广泛接受的全球海平面数据相协调。此外，最近对德克萨斯州墨西哥湾沿岸的研究表明，全新世中期海平面高度比现在的海平面高2米。这项研究将使用迄今尚未应用于墨西哥湾沿岸的技术，从过去~7000年的密西西比三角洲东部和西部边缘收集新的海平面指标。该方法是基于使用基底泥炭，这些泥炭覆盖在无压实的更新世基底上，可以与平均海平面相关。从基底泥炭样品和加速器质谱测定的14C中选择与平均海平面相关的具有生态指示意义的植物宏观化石。此外，将测量稳定的碳同位素比率，以优先选择指示微咸或盐水条件的样品。该项目的主要成果将构成密西西比三角洲的新的相对海平面曲线，该曲线将(1)评估作为海平面指标的基底泥炭的垂直分辨率及其重建微妙的、与气候有关的海平面上升的潜力；(2)验证全新世海平面上升“阶梯”模式的有效性；(3)确定不同构造沉降速率的产状和大小；(4)确定墨西哥湾沿岸是否可能出现过全新世中期的海平面高点（如果有的话，在这一下沉区域，海平面上升速度明显下降）。这一结果将对密西西比三角洲沿海湿地损失的未来预测具有重要意义。

Rates of Holocene Relative Sea-level Rise and Differential Crustal Movements in the Mississippi DeltaTorbjorn TornqvistEAR 0074065Accelerated sea level rise due to global warming is expected to become a huge problem in the Mississippi Delta, where dramatic loss of coastal wetlands at rates of ~100 km2 per year is already taking place. To facilitate future wetland management strategies, a proper understanding of natural rates of sea-level rise and its spatial variability due to differential tectonic subsidence rates is indispensable. Recent studies in other major deltaic plains have shown that spatially nonuniform crustal movements can be considerable. Presently available sea-level data from the Mississippi Delta do not resolve differential tectonic subsidence rates, whereas most Holocene relative sea-level curves for the Gulf Coast exhibit a conspicuous, but commonly mutually conflicting, 'stair-step' pattern of alternating sea-level stillstands and rapid rises. The scale of these phenomena is an order of magnitude larger that the subtle (1 m), supposedly climate-related sea-level fluctuations that have been inferred elsewhere, and they are difficult to reconcile with widely accepted global sea-level data. Furthermore, recent work on the Texas Gulf Coast has proposed a middle Holocene sea-level highstand of 2 m above present sea level.This study will collect new sea-level indicators from the eastern and western margins of the Mississippi Delta for the last ~7000 years, using techniques that have so far not been applied to the Gulf Coast. The approach is based on the use of basal peats that overlie a compaction-free Pleistocene basement and can be related to mean sea level. Ecologically indicative plant macrofossils that can be related to mean sea level will be selected from basal-peat samples and 14C dated by accelerator mass spectrometry. In addition, stable carbon isotope ratios will be measured to preferentially select samples indicative of brackish or saline conditions.The premier outcome of this project will constitute new relative sea-level curves for the Mississippi Delta that will (1) assess the vertical resolution of basal peats as sea-level indicators and their potential for reconstructing subtle, climate-related sea-level rise; (2) test the validity of the inferred 'stair-step' pattern of Holocene sea-level rise; (3) determine the occurrence and magnitude of differential tectonic subsidence rates; and (4) establish whether a middle Holocene sea-level highstand is likely to have occurred on the Gulf Coast (if so, represented in this subsiding area by a distinct reduction in the rate of sea-level rise). The results will be important for future projections of coastal wetland loss in the Mississippi Delta.