Oceanography | Vol. 38, No. 2
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environment. The real-time data streams permit continuous
monitoring of seismic events and seafloor deformation, pro
viding insights into processes influencing environmental sta
bility and enabling timely responses to significant events. Since
2018, heightened seismicity has been observed by Krauss et al.
(2023), mirroring precursors to past diking events (1999–2005).
This culminated in a notable increase in activity in March 2024,
including an M4.1 earthquake and periods with up to 200 events
per hour, suggesting the segment may be approaching the next
diking event, prompting the scientific community to meet in
November 2024 to prepare for a rapid response to a major per
turbation of the system.
Understanding the chemical environment driving these eco
systems is critical. Due to the harsh environment of hydrother
mal venting regions, geochemical sensors for measuring the
continuous temporal variability of the chemistry of fluid emis
sions are very limited, and scientific research has relied predom
inantly on laboratory analysis of discrete samples obtained on
scientific expeditions. To obtain a continuous time series at high
temperature vents, ONC employed a cable-connected BARS
to measure temperature, resistivity, and redox potential (eH)
of the vent fluids in situ (Table 1). With discrete samples taken
at the beginning of the deployment and at the time of recov
ery, the continuous time series of the sensors’ measurements are
used to infer changes in fluid chemistry. However, these sensors
reside in black smoker vents with 300°–350°C fluid emission
and often do not last a full year between maintenance expedi
tions. Another method to improve time resolution of the vari
ability of chemical fluxes is to remotely collect discrete sam
ples. Currently, a serial gas tight sampler is deployed alongside a
BARS. Its containers can be remotely triggered to collect a time
series of 12 vent fluid samples (Seyfried et al., 2022). The timing
of the sampling is adapted to changes in seismicity or vent fluid
temperatures, allowing correlation between specific geological
events and vent fluid chemistry.
The current period of heightened seismicity marks a critical
phase for the evolution of the Endeavour Segment. It presents a
rare opportunity to observe a potential dike intrusion or spread
ing event that would offer valuable data for refining models of
mid-ocean ridge processes. Studying the Endeavour Segment,
with its intermediate spreading rate characteristics, provides a
key comparison point between fast- and slow-spreading sys
tems across the globe and other intermediate spreading cen
ters (e.g., the Galápagos Spreading Center). An impending tec
tonic event may cause significant shifts in hydrothermal output
(heat and chemistry), providing a natural experiment to study
the resilience and adaptive responses of the specialized vent
communities within the MPA. To better capture such an event,
Dalhousie University and the University of Washington, in part
nership with ONC, enhanced observatory capabilities by deploy
ing five autonomous ocean bottom seismometers in summer
2024; an additional 20 ocean bottom seismometers (including
replacements for the 2024 units) are scheduled for deployment in
summer 2025. This denser network will improve detection and
location of seismicity, providing crucial data for understanding
geological and tectonic processes and their impacts on hydro
thermal vent ecosystems. It will inform future MPA management
strategies regarding natural and anthropogenic disturbances.
OCEANIC ENVIRONMENT
Changes in and redistribution of heat and chemical fluxes from
the vent fields along Endeavour’s axial valley alter seafloor char
acteristics, affecting benthic ecosystems as well as the overlying
water column and the pelagic ecosystem it hosts. Not only is
the seawater chemistry directly altered, but changes in the heat
flux from hydrothermal venting affects the local ocean circula
tion through changes in buoyancy input from the rising hydro
thermal plumes. On an axial valley scale, the rising plume gen
erates inflow near the seafloor toward the hydrothermal vents,
which facilitates the retention of vent field larvae and plankton.
Conversely, the rising plume can also entrain planktonic organ
isms, moving them up into the water column where along-axis
currents can relocate them to vent sites with more retentive cir
culation, or higher up in the water column where they can be
swept away by ambient ocean currents to less hospitable ocean
environments (Thomson et al., 2003). If the organisms have the
ability to swim vertically or alter their buoyancy, they can use
this circulation to move to a preferred location. There is obser
vational evidence of larvae exhibiting this type of behavior at
other vents sites (Mullineaux et al., 2013). On the segment scale,
the off-axis propagation of the plume alters the chemistry of the
ocean (Coogan et al., 2017; Beaupre-Olsen et al., 2025) and has
a marked impact on the overlying pelagic ecosystem, enhancing
secondary productivity (Burd and Thomson, 2015).
Estimating the flux of hydrothermal fluid and heat along the
Endeavour Segment has generally been conducted by observing
water property anomalies, either by dense shipborne or autono
mous underwater vehicle (AUV) sampling. These observations
are inverted to estimate flux using the known temperature of the
vent fluid as it leaves the seafloor (Kellogg, 2011), resulting in an
overall value of heat flux over the time window of the repeated
surveys. With yearly AUV surveys (2004, 2005, 2006), Kellogg
and McDuff (2010) identified a transient anomaly over the Salty
Dawg vent field, suggesting that there is spatial and temporal vari
ability in hydrothermal flux; however, their temporal resolution
made it difficult to determine both the subseafloor causes and the
water column effects. As an alternative to annual AUV surveys
with their inherent coarse time resolution, four moorings of cur
rent meters and water property sensors (CTDs) were installed in
the axial valley of the Endeavour Segment. The array is designed
to utilize the “sea breeze effect” caused by the rising buoyant
plume. This effect relates horizontal currents to the intensity of
heat flux from the hydrothermal venting (Thomson et al., 2003);
therefore, variability in hydrothermal heat flux is continuously