North Atlantic Deep Water (NADW), the return flow component of the Atlantic Meridional Overturning Circulation (AMOC), is a major inter-hemispheric ocean water mass with strong climate effects but the evolution of its source components on million-year timescales is poorly known. Today, two major NADW components that flow southward over volcanic ridges to the east and west of Iceland are associated with distinct contourite drift systems that are forming off the coast of Greenland and on the eastern flank of the Reykjanes (mid-Atlantic) Ridge. Here we provide direct records of the early history of this drift sedimentation based on cores collected during International Ocean Discovery Programme (IODP) Expeditions 395C and 395. We find rapid acceleration of drift deposition linked to the eastern component of NADW, known as Iceland-Scotland Overflow Water at 3.6 million years ago (Ma). In contrast, the Denmark Strait Overflow Water feeding the western Eirik Drift has been persistent since the Late Miocene. These observations constrain the long-term evolution of the two NADW components, revealing their contrasting independent histories and allowing their links with climatic events such as Northern Hemisphere cooling at 3.6 Ma, to be assessed.

Mysticete whales have bilateral bony ear complexes (tympanoperiotic complexes) that amplify low frequency vibrations in proximity to their vocalization ranges. Understanding the functional mechanics would enable precise predictions of mysticete hearing sensitivity, which is currently unknown. We conducted experiments on a juvenile and an adult gray whale skull from deceased animals to measure the vibrational dynamics between the tympanic bullae and the skull. Relative motions between bullae and skull indicate sound transfer to the inner ear. For the juvenile, assessments were performed on (1) a 3D-printed plastic skull-replica, (2) the original skull after much of the soft tissue had been removed by dissection, and (3) the denuded skull after hydrogen peroxide was used to erode the remaining soft tissues. We excited vibrations in the juvenile skull underwater by projecting sound in a test pool, ranging from 170-1000 Hz. Additionally, we measured in-air vibrations of the plastic, denuded, and adult skulls using a mechanical shaker to drive vibrations anteroposteriorly (rostrum-to-tail) from 150-1000 Hz. The results consistently showed amplification of vibrations at the tympanic bullae compared to the base of the skull, demonstrating a mechanism by which low-frequency sound is transferred from the environment, through the skull, to the inner ear.

The tympanoperiotic complex (TPC) bones of the fin whale skull were studied using experimental measurements and simulation modeling to provide insight into the low frequency hearing of these animals. The study focused on measuring the sounds emitted by the left and right TPC bones when the bones were tapped at designated locations. Radiated sound was recorded by eight microphones arranged around the tympanic bulla. A finite element model was also created to simulate the natural mode vibrations of the TPC and ossicular chain, using a 3D mesh generated from a CT scan. The simulations produced mode shapes and frequencies for various Young's modulus and density values. The recorded sound amplitudes were compared with the normal component of the simulated displacement and it was found that the modes identified in the experiment most closely resembled those found with Young's modulus for stiff and flexible bone set to 25 and 5 GPa, respectively. The first twelve modes of vibration of the TPC had resonance frequencies between 100Hz and 6kHz. Many vibrational modes focused energy at the sigmoidal process, and therefore the ossicular chain. The resonance frequencies of the left and right TPC were offset, suggesting a mechanism for the animals to have improved hearing at a range of frequencies as well as a mechanism for directionality in their perception of sounds.

Computational models serve as useful complements to physical experiments, but they require validation to build confidence in their applicability. This study outlines the validation of biomechanical models for mysticete sound reception, specifically using experiments involving an instrumented gray whale skull exposed to underwater sound. Detailed descriptions of the models are provided. The models were evaluated using a set of similarity metrics applied to both measured and computed frequency response functions. While high-quality agreement was not achieved, the models corresponded reasonably well with observed experimental data. A sensitivity analysis examined the models' responses to variations in input material properties. Although these changes in material properties influenced model response, they accounted for only modest changes in similarity. A more significant challenge to achieving higher accuracy was the mismatch between the acoustic waves generated in experiments and the models' assumption of plane wave loading. Despite this, the models successfully captured important biomechanical behavior, such as the enhancement of motion of the tympanic bullae relative to the basicranium. Model validation remains an ongoing endeavor, and this study represents an initial step.