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

密西西比三角洲全新世相对海平面上升速率和地壳差异运动Torbjorn Tornqvistoro 0074065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，那里的沿海湿地已经以每年约100平方公里的速度急剧丧失。 为了促进未来的湿地管理战略，海平面上升的自然率和空间变异性，由于不同的构造沉降率的正确理解是必不可少的。 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是相当大的。 目前可用的海平面数据从密西西比三角洲没有解决差异构造沉降率，而大多数全新世相对海平面曲线墨西哥湾沿岸表现出明显的，但通常相互冲突，“阶梯”模式的交替海平面stillstands和快速上升。 这些现象的规模比其他地方推断的微妙的（1米），据推测与气候有关的海平面波动大一个数量级，它们很难与广泛接受的全球海平面数据相一致。 此外，最近的工作在得克萨斯州墨西哥湾沿岸提出了一个中全新世海平面高海拔2米以上，现在的海平面。这项研究将收集新的海平面指标，从东部和西部边缘的密西西比三角洲在过去的~7000年，使用的技术，到目前为止还没有被应用到墨西哥湾沿岸。 该方法是基于使用的基底泥炭覆盖的无植被更新世基底，可以与平均海平面。 从基泥炭样品中选择与平均海平面相关的植物化石，并通过加速器质谱法测定14 C年龄。 此外，还将测量稳定的碳同位素比值，以便优先选择指示微咸或含盐条件的样品。该项目的主要成果将为密西西比三角洲绘制新的相对海平面曲线，该曲线将（1）评估作为海平面指示器的基泥炭的垂直分辨率及其重建与气候有关的微妙海平面上升的潜力;（2）检验全新世海平面上升的“阶梯式”模式的有效性：（3）确定差异构造沉降速率的发生和大小;以及（4）确定墨西哥湾沿岸是否可能发生过中全新世海平面高位（如果是的话，在这个沉降区，海平面上升的速度明显减少）。 研究结果将对未来预测密西西比三角洲沿海湿地的损失具有重要意义。

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.