Collaborative Research: Are Buried Paleochannels Effective Reactors for Water and Solute Transport in a Deltaic Subterranean Estuary?

合作研究：地下三角洲河口埋藏古河道是水和溶质运移的反应器吗？亚历山大科尔克，路易斯安那州大学海洋财团卡伦约翰森，杜兰大学杰伊凯布尔，北卡罗来纳大学教堂山河流三角洲是地球上最大的矿物和富含有机物的沉积物储存库之一。 它们的低起伏和靠近海洋也使它们成为研究沿海生态系统如何应对未来几十年气候变化和海平面上升的最佳系统之一。 以前的大量研究集中在三角洲在碳、营养物质、金属和其他元素循环中的作用。 然而，这项研究很少调查的地下连接之间的主要河流和邻近的海湾和湿地。 与河流三角洲相关的沿海海湾的水文模型表明，进入这些系统的淡水比预期的要多。 这项研究将调查是否这个失踪的组成部分是地下水排放通过埋藏在这些三角洲的古代渠道。 现代三角洲包括海湾和河口，沉积物和沼泽，形成和废弃的河流在整个全新世改变路线。 随着时间的推移，沙底海湾被掩埋，可能只在表面留下它们以前存在的痕迹。 这些埋藏的渠道通常保持与主要河流的水力连接，因此在每年春季/夏季高水位期间作为排放到邻近海湾的管道。 在密西西比河三角洲（MRD）系统内，由于防洪堤可能产生比相邻海湾水位高出15至22英尺的水位（即水头），这种水力连接可能会增强20至40英尺。 这项研究的核心假设是，三角洲中埋藏的古代河道充当了一个巨大的地下河口网络，在将地下水、营养物质和一些金属输送到三角洲海湾并最终输送到海洋方面发挥了关键作用。 该研究解决了几个主要问题：三角洲海底地下水排放量是否足以满足当前三角洲水文和地球化学收支？ 古河道网络向沿海海洋输送水和元素质量通量的有效性如何？ 这些问题将通过地球物理调查来解决，这些调查可以产生三角洲沉积物结构及其盐度的图像，通过声纳和电导率来确定。 这些研究将与地球化学示踪剂（如盐、放射性和稳定同位素）的研究一起进行，并详细研究河流与古河道系统（如压力计、压力记录仪、流网）之间的水力梯度。 使用组装的水文地质流动模式和速率的理解，我们将估计与季节性地下水流通过这个古河道/地下河口网络（如N，C，P，Si，Fe）的地球化学质量通量。 研究结果将有助于更好地理解三角洲的地质在水文和化学方面所起的作用，以及水、营养物质和金属的通量如何随空间和时间而变化。 这项工作将增加对三角洲和其他沿海生态系统功能的理解，特别是考虑到未来50年海平面上升的预测，河流长期以来一直被认为在全球化学循环中发挥着重要作用。尽管认识到这一点，但人们对这些化学循环在海底以下如何运作以及这对沿海地区化学的影响知之甚少。 拟议的研究将审查这些过程中的密西西比河三角洲。 这个三角洲位于北美最大的河流的河口，是美国最重要的经济途径之一的入口。 密西西比河三角洲在科学研究、政府水质监测和利益相关者参与方面也有着悠久的历史。 这为该小组提供了极好的基线信息，并有机会将调查结果与社会需求联系起来。 这项工作将有助于三个研究生和几个本科生的教育。 这些学生将在领先的研究型大学学习，并有机会在海洋实验室工作。

COLLABORATIVE RESEARCH:ARE BURIED PALEOCHANNELS EFFECTIVE REACTORS FOR WATER AND SOLUTE TRANSPORT IN THE DELTAIC SUBTERRANEAN ESTUARY?Alexander Kolker, Louisiana Universities Marine ConsortiumKaren Johannesson, Tulane UniversityJaye Cable, University of North Carolina-Chapel HillRivers deltas are one of the largest stores of minerals and organic-rich sediments on earth. Their low relief and close proximity to the ocean also make them one of the best systems to study how coastal ecosystems will respond to climate change and sea-level rise in the coming decades. A wealth of previous studies focused on the role deltas play in cycling carbon, nutrients, metals and other elements. However, very little of this research has investigated the subsurface connections between the main river and adjacent bays and wetlands. Hydrological models of coastal bays associated with river deltas suggest more freshwater is entering these systems than expected. This study will investigate whether this missing component is groundwater discharge through buried ancient channels in these deltas. Modern deltas consist of bays and bayous, sedimentary deposits, and marshes formed and abandoned as the river changed course throughout the Holocene. Over time, sandy-bottom bayous are buried and may leave only a trace of their former existence at the surface. These buried channels commonly retain a hydraulic connection to the main river, thus acting as a conduit for discharge to adjacent bays during the annual spring/summer high river stage. Within the Mississippi River Delta (MRD) system, this hydraulic connection may be enhanced by 20 to 40 ft as a consequence of flood control levees which may produce a river stage (i.e. head) as much as 15 to 22 ft above the adjacent bay water levels. The central hypothesis of this research is that buried ancient channels in deltas act as a vast network of subterranean estuaries, which play a critical role in the transport of groundwater, nutrients, and some metals to deltaic bays and ultimately the ocean. The research addresses several major questions: Is deltaic submarine groundwater discharge sufficient to satisfy current deltaic hydrologic and biogeochemical budgets? How effective are paleochannel networks in the delivery of water and elemental mass fluxes to the coastal ocean? These questions will be addressed using geophysical surveys that can produce images of the structure of the delta sediments and their salinity, as determined through sonars and electrical conductivity. These studies will be conducted in concert with studies of geochemical tracers (e.g. salt, radioactive and stable isotopes) and detailed a study of hydraulic gradients between the river and a paleochannel system (e.g. piezometers, pressure loggers, flow nets). Using the assembled understanding of hydrogeologic flow patterns and rates, we will estimate the biogeochemical mass fluxes associated with seasonal groundwater flow through this paleochannel/subterranean estuary network (e.g. N, C, P, Si, Fe). Results should improve the understanding of the role that the delta's geology plays in its hydrology and chemistry and how fluxes of water, nutrients and metals vary over space and time. This work will then add to the understanding of how deltas and other coastal ecosystems function, particularly in light of sea-level rise predictions for the next 50 years.Rivers have long been recognized a playing an important role global chemical cycles. Despite this recognition, relatively little is know about how these chemical cycles function below the sea floor, and the implications this has for the chemistry of the coastal zone. The proposed study will examine these processes in the Mississippi River Delta. This delta sits at the mouth of the largest river in North America, and is the entry point to one of the most important economic pathways in the United States. The Mississippi River Delta also has a long history of scientific research, government water quality monitoring, and stakeholder involvement. This provides the team with excellent baseline information and opportunities to link findings to the needs of society. The work will contribute to the education of three graduate students and several undergraduate students. These students will study at leading research universities and have the opportunity to work at a marine laboratory.