Exp. 406 New England Shelf Hydrogeology

航海概要

テーマ

New England Shelf Hydrogeology

• プロポーザル #637
• Scientific Prospectus: To be published

ECORDのページ＞＞こちら

航海予定期間

Offshore phase (乗船): 2024年6月～8月末 (最長90日間)
Onshore Science Party (航海後の陸上分析): 2024年末または2025年初め（詳細な日程は未定、最長4週間）
MARUM（ドイツのブレーメン大学海洋環境科学センター）にて実施

※応募される際、「Onshoreのみ参加希望」もしくは「Offshoreとonshoreの両方に参加希望」をご記入ください。「Offshoreのみの参加」という選択肢はありませんのでご注意ください。

掘削船

Mission Specific Platform (TBD)

乗船/下船地

TBD

掘削地点

 New England Shelf

科学目的

世界各地の大陸棚では沖合の堆積層に淡水が浸入していることが知られています。ニューイングランド沖の大陸棚はその代表的な場所の一つであり、低塩分濃度の地下水が100 km以上の沖合まで広がっていることが知られています。このような海底下への顕著な淡水の涵養（recharge）のメカニズムを説明するいくつかの水理学モデルを検証するために、本航海はニューイングランド、マサチューセッツ州のマーサズ・ヴィニヤード島沖の大陸棚浅海域で４地点の掘削を行います。コア採取、検層、流体採取、ガス採取によって、流体の分布、組成、水理地質を明らかにし、大陸棚における地下水の動態、ならびに微生物、炭素循環、栄養塩循環との関与の解明を目指します。

In many coastal settings worldwide, the distribution of freshwater within continental shelf sediments is far out of equilibrium with modern sea level. One of the most remarkable examples is found on the Atlantic continental shelf off New England where groundwater within shallow Pliocene-Pleistocene sand aquifers over 100 km offshore has low salinity (3000 mg/l or less). On Nantucket Island, a 514m deep borehole penetrating the entire Cretaceous-Tertiary sedimentary package shows considerable variations in salinity with extremely fresh (<1000 mg/l) water in sand aquifers, higher salinity (30-70% of seawater) in thick clay/silt layers, and intermediate-to-low salinity in thin confining units.

IODP Expeditions 313 and 317 also showed abrupt freshwater-saltwater boundaries linked to lithology. This demonstrates the disequilibrium nature of such systems; diffusion tends to eliminate such patterns. Pore fluid within Pleistocene to upper Cretaceous sands beneath Nantucket Island is also found to be modestly overpressured, ~4m relative to the local water table.
We hypothesize that the rapid incursion of freshwater on the continental shelf in New England could have been caused by one or more of the following mechanisms:
1) meteoric recharge during Pleistocene sea-level lowstands including vertical infiltration of freshwater associated with local flow cells on the shelf;
2) sub-ice-sheet recharge during the last glacial maximum;
3) recharge from pro-glacial lakes.
We further hypothesize that the overpressures could be due to
1) Pleistocene sediment loading;
2) fluid density differences associated with emplacement of a thick freshwater lens over saltwater (analogous to excess pressures in the gas legs of petroleum reservoirs).
We argue these different recharge mechanisms and overpressure models can be distinguished through drilling, coring, logging, and fluid sampling. Noble gas and environmental isotope data will be necessary to completely evaluate recharge models.

This work will extend our understanding of the current and past states of fluid composition, pressure, and temperature in continental shelf environments. It will help better constrain rates, directions, and mechanisms of groundwater flow and chemical fluxes in continental shelf systems. It will contribute to the development of new tools for measuring freshwater resources in marine environments. The apparent transient nature of continental shelf salinity patterns could have important implications for microbial processes and long-term fluxes of carbon, nitrogen, and other nutrients to the ocean. Successful drilling will test process-based models for shelf freshwater off New England. These models can then be applied to other shelf freshwater systems around the world.

詳細についてはCall for Applications、ECORDのページをご参照ください。

共同首席研究者

Brandon Dugan (Colorado School of Mines, USA)
Karen Johannesson (University of Massachusetts Boston, USA)

J-DESCからの乗船研究者

乗船者選考中