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

合作研究：埋藏的古河道是三角洲地下河口水和溶质运输的有效反应堆吗？路易斯安那大学海洋协会的Alexander Kolker，杜兰大学的Karen Johannesson，北卡罗来纳大学的Jaye Cable，北卡罗来纳大学教堂山河三角洲是地球上最大的矿物和富含有机物的沉积物之一。它们的地势低，靠近海洋，也使它们成为研究沿海生态系统在未来几十年将如何应对气候变化和海平面上升的最佳系统之一。之前的大量研究集中在三角洲在碳、营养、金属和其他元素循环中所起的作用。然而，这项研究很少调查主要河流与邻近海湾和湿地之间的地下联系。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。这项研究将调查这些三角洲中缺失的部分是否是通过埋藏的古河道排放的地下水。现代三角洲包括海湾和海湾沉积，以及在整个全新世河流改道时形成和废弃的沼泽。随着时间的推移，沙底海湾被掩埋，可能只会在地表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，从而在每年春夏高潮期充当向邻近海湾排放的管道。在密西西比河三角洲(MRD)系统内，由于防洪堤坝可能产生比邻近海湾水位高达15至22英尺的河段(即水头)，这种水力连接可能会增强20至40英尺。这项研究的中心假设是，三角洲中被埋藏的古河道就像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中发挥着关键作用。这项研究解决了几个主要问题：三角洲海底地下水排放是否足以满足当前三角洲水文和生物地球化学预算？在向沿海海洋输送水和元素质量通量方面，古河道网络的效率如何？这些问题将通过地球物理调查加以解决，地球物理调查可以产生通过声纳和电导率确定的三角洲沉积物的结构及其盐度的图像。这些研究将与地球化学示踪研究(如盐、放射性和稳定同位素)一起进行，并详细研究河流与古河道系统之间的水力梯度(如压力计、压力记录仪、流网)。利用对水文地质流动模式和速率的综合了解，我们将估计与季节性地下水通过这个古河道/地下河口网络(如N、C、P、Si、Fe)有关的生物地球化学质量通量。研究结果应该有助于加深对三角洲地质在其水文和化学中所扮演的角色，以及水、营养物质和金属的通量如何随空间和时间变化的理解。这项工作将增进对三角洲和其他沿海生态系统如何发挥作用的理解，特别是根据对未来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。尽管认识到了这一点，但对于这些化学循环在海底是如何运作的，以及这对沿海地区化学的影响，人们知之甚少。这项拟议的研究将研究密西西比河三角洲的这些过程。这个三角洲位于北美最大的河流的入海口，是美国最重要的经济通道之一的入口点。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。这为团队提供了极好的基线信息和机会，将调查结果与社会需求联系起来。这项工作将有助于培养三名研究生和几名本科生。这些学生将在一流的研究型大学学习，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.