Long‐Term Observations of Subseafloor Temperatures and Pressures in a Low‐Temperature, Off‐Axis Hydrothermal System in North Pond on the Western Flank of the Mid‐Atlantic Ridge.

Basement formation pressures and temperatures were recorded from 1997 to 2017 in four sealed‐hole observatories in North Pond, an isolated ∼8 × 15 km sediment pond surrounded by thinly sedimented basement highs in 7–8 Ma crust west of the Mid‐Atlantic Ridge at ∼23°N. Two observatories are located ∼1 km from the southeastern edge of North Pond where sediment thickness is ∼90 m; the other two are ∼1 km from the northeastern edge where sediment thickness is 40–50 m. Sediments are up to 200 m thicker in the more central part of the pond. The borehole observations, along with upper basement temperatures estimated from seafloor heat flux measurements, provide constraints on the nature of low‐temperature ridge‐flank hydrothermal circulation in a setting that may be typical of sparsely sedimented crust formed at slow spreading ridges. Relative to seafloor pressures, basement formation pressures are modestly positive and increase with depth, except for a slight negative differential pressure in the shallowest 30–40 m in one northeastern hole. Although the observatory pairs are ∼6 km apart, the lateral pressure gradient in basement between them is very small. Formation pressure responses to seafloor tidal loading are consistent with high basement permeability that allows for vigorous low‐temperature circulation with low lateral pressure gradients. In contrast, there is significant lateral variability in upper basement temperatures, with highest values of ∼12.5°C beneath the thickly sedimented southwest section, lower values near the edges, and lowest values near the southeast edge. The results are key to assessing past and recent models for the circulation system. Plain Language Summary: Low temperature hydrothermal systems in young oceanic seafloor have important integrated thermal and chemical effects, but flow patterns are weakly constrained by direct observations and are poorly understood, especially in sparsely sedimented settings that are characteristic of much of the world's seafloor. We present 20 years of observations of seafloor and formation pressures and temperatures recorded by sealed borehole observatories beneath a North Atlantic ridge‐flank sedimented basin ("North Pond") surrounded by sparsely sedimented basement highs—the first such long‐term data from this common type of setting. The observations indicate that there is a vigorous low‐temperature hydrothermal flow system beneath the sediments of North Pond as previously concluded, but lateral pressure gradients in uppermost basement are very low; these observations require the permeability of uppermost basement to be very high. Our determinations of uppermost basement temperatures show greater variations than previously inferred, with the highest temperatures beneath the most thickly sedimented section. This is inconsistent with prior conceptual models of unidirectional hydrothermal flow in uppermost basement beneath the sediment pond, but more supportive of recent models with upflow beneath the most thickly sedimented section of the pond, accompanied by lateral flow toward the pond perimeter where mixing with seawater occurs. Key Points: Formation pressures in igneous basement beneath North Pond sediments are positive except in the upper few tens of meters near the pond edgeLateral pressure gradients in upper igneous basement are very low, implying very high permeabilities (∼10−10 m2) and vigorous flowBasement temperatures are highest beneath the thickest sediments, unsupportive of prior models for unidirectional flow in igneous basement [ABSTRACT FROM AUTHOR]