June 2025

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Oceanography | Vol. 38, No. 2

16

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

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