A multi-decadal record of oceanographic changes of the past ~165 years (1850-2015 AD) from Northwest of Iceland.

Extending oceanographic data beyond the instrumental period is highly needed to better characterize and understand multi-decadal to centennial natural ocean variability. Here, a stable isotope record at unprecedented temporal resolution (1 to 2 years) from a new marine core retrieved off western North Iceland is presented. We aim to better constrain the variability of subsurface, Atlantic-derived Subpolar Mode Water (SPMW), using near surface-dwelling planktic foraminifera and Arctic Intermediate Water (AIW) mass changes using benthic foraminifera over the last ~165 years. The reconstruction overlaps in time with instrumental observations and a direct comparison reveals that the δ18O record of Neogloboquadrina pachyderma is reliably representing temperature fluctuations in the SPMWs. Trends in the N. pachyderma δ13C record match the measured phosphate concentration in the upper 200 m on the North Icelandic Shelf well. Near surface-dwelling foraminifera trace anthropogenic CO2 in the Iceland Sea by ~ 1950 ± 8, however, a reduced amplitude shift in the Marine Suess effect is identified. We argue that this is caused by a contemporary ongoing increase in marine primary productivity in the upper ocean due to enhanced Greenland's freshwater discharge that has contributed to a nutrient-driven fertilization since the 1940s/50s (Perner et al., 2019). Multi-decadal variability is detected. We find that the 16-year periodicity evident in SPMW and AIWs based on the δ18O of N. pachyderma and M. barleeanum is a signal of SST anomalies propagated into the Nordic Seas via the Atlantic inflow branches around Iceland. Spectral analyses of the planktic foraminiferal δ13C signal indicate intermittent 30-year cycles that are likely reflecting the ocean response to atmospheric variability, presumably the East Atlantic Pattern. A long-term trend in benthic δ18O suggests that Atlantic-derived waters are expanding their core within the water column from the subsurface into deeper intermediate depths towards the present day. This is a result of increased transport by the North Icelandic Irminger Current to the North Iceland Shelf over the historical era.

A 50-Year Journey from Phosphate to Autonomous Underwater Vehicles.

This narrative is a personal account of my evolution as a student of phytoplankton and the ocean. Initially I focused on phytoplankton nutrient physiology and uptake, later switching to photosynthetic physiology. Better models of photosynthesis naturally require a better understanding of spectral underwater light fields and absorption coefficients, which precipitated my involvement in the nascent field of bio-optical oceanography. Establishment of the now 34-year-old summer graduate course in ocean optics, which continues to attract students from around the globe, is a legacy of my jumping into optics. The importance of social interactions in advancing science cannot be underestimated; a prime example is how a TGIF gathering led to my immersion in the world of autonomous underwater vehicles for the past two decades of my career. Working with people who you like and respect is also critical; I believe collegial friendship played a major role in the great success of the 2008 North Atlantic Bloom Experiment.

GEOTRACES: Accelerating Research on the Marine Biogeochemical Cycles of Trace Elements and Their Isotopes.

The biogeochemical cycles of trace elements and their isotopes (TEIs) constitute an active area of oceanographic research due to their role as essential nutrients for marine organisms and their use as tracers of oceanographic processes. Selected TEIs also provide diagnostic information about the physical, geological, and chemical processes that supply or remove solutes in the ocean. Many of these same TEIs provide information about ocean conditions in the past, as their imprint on marine sediments can be interpreted to reflect changes in ocean circulation, biological productivity, the ocean carbon cycle, and more. Other TEIs have been introduced as the result of human activities and are considered contaminants. The development and implementation of contamination-free methods for collecting and analyzing samples for TEIs revolutionized marine chemistry, revealing trace element distributions with oceanographically consistent features and new insights about the processes regulating them. Despite these advances, the volume and geographic coverage of high-quality TEI data by the end of the twentieth century were insufficient to constrain their global biogeochemical cycles. To accelerate progress in this field of research, marine geochemists developed a coordinated international effort to systematically study the marine biogeochemical cycles of TEIs-the GEOTRACES program. Following a decade of planning and implementation, GEOTRACES launched its main field effort in 2010. This review, roughly midway through the field program, summarizes the steps involved in designing the program, its management structure, and selected findings.

A Conversation with Walter Munk.

In this interview, Carl Wunsch talks with Walter Munk about his career in oceanography; his relationships with scientists such as Harald Sverdrup, Roger Revelle, Walfrid Ekman, Carl Rossby, Carl Eckart, Henry Stommel, and G.I. Taylor; technological advances over the decades; and his thoughts on the future of the field.

Passing the Baton to the Next Generation: A Few Problems That Need Solving.

This is a personal account of some of the people and factors that were important in my career in chemical oceanography. I also discuss two areas of oceanographic research and training that I think need more attention. The first is how the difficulty in getting appropriate samples hampers our ability to fully understand biogeochemical processes in the sea. I have worked on dissolved materials, suspended and sinking particles, and sediments in lakes, oceans, rivers, and aerosols. Sample collection problems affect all those areas, although to different degrees. Second, I discuss a few of the issues that I most worry about with regard to graduate education in oceanography, among them an apparent decrease over the past several decades in the ability of many beginning students to write clearly and think logically.

Ocean Data Assimilation in Support of Climate Applications: Status and Perspectives.

Ocean data assimilation brings together observations with known dynamics encapsulated in a circulation model to describe the time-varying ocean circulation. Its applications are manifold, ranging from marine and ecosystem forecasting to climate prediction and studies of the carbon cycle. Here, we address only climate applications, which range from improving our understanding of ocean circulation to estimating initial or boundary conditions and model parameters for ocean and climate forecasts. Because of differences in underlying methodologies, data assimilation products must be used judiciously and selected according to the specific purpose, as not all related inferences would be equally reliable. Further advances are expected from improved models and methods for estimating and representing error information in data assimilation systems. Ultimately, data assimilation into coupled climate system components is needed to support ocean and climate services. However, maintaining the infrastructure and expertise for sustained data assimilation remains challenging.

Global Ocean Integrals and Means, with Trend Implications.

Understanding the ocean requires determining and explaining global integrals and equivalent average values of temperature (heat), salinity (freshwater and salt content), sea level, energy, and other properties. Attempts to determine means, integrals, and climatologies have been hindered by thinly and poorly distributed historical observations in a system in which both signals and background noise are spatially very inhomogeneous, leading to potentially large temporal bias errors that must be corrected at the 1% level or better. With the exception of the upper ocean in the current altimetric-Argo era, no clear documentation exists on the best methods for estimating means and their changes for quantities such as heat and freshwater at the levels required for anthropogenic signals. Underestimates of trends are as likely as overestimates; for example, recent inferences that multidecadal oceanic heat uptake has been greatly underestimated are plausible. For new or augmented observing systems, calculating the accuracies and precisions of global, multidecadal sampling densities for the full water column is necessary to avoid the irrecoverable loss of scientifically essential information.