MIS 5e sea-level history along the Pacific coast of North America
Authors: Muhs Daniel R. 
Published: March 2022

Abstract. The primary last interglacial, marine isotope substage
(MIS) 5e records on the Pacific coast of North America, from Washington
(USA) to Baja California Sur (Mexico), are found in the deposits of
erosional marine terraces. Warmer coasts along the southern Golfo de
California host both erosional marine terraces and constructional coral reef
terraces. Because the northern part of the region is tectonically active,
MIS 5e terrace elevations vary considerably, from a few meters above sea
level to as much as 70 m above sea level. The primary paleo-sea-level
indicator is the shoreline angle, the junction of the wave-cut platform with
the former sea cliff, which forms very close to mean sea level. Most areas
on the Pacific coast of North America have experienced uplift since MIS 5e
time, but the rate of uplift varies substantially as a function of tectonic
setting. Chronology in most places is based on uranium-series ages of the
solitary coral Balanophyllia elegans (erosional terraces) or the colonial corals Porites and
Pocillopora (constructional reefs). In areas lacking corals, correlation to MIS 5e often
can be accomplished using amino acid ratios of fossil mollusks, compared to
similar ratios in mollusks that also host dated corals. Uranium-series (U-series) analyses of
corals that have experienced largely closed-system histories range from
∼124 to ∼118 ka, in good agreement with ages
from MIS 5e reef terraces elsewhere in the world. There is no geomorphic,
stratigraphic, or geochronological evidence for more than one high-sea stand
during MIS 5e on the Pacific coast of North America. However, in areas of
low uplift rate, the outer parts of MIS 5e terraces apparently were
re-occupied by the high-sea stand at ∼100 ka (MIS 5c),
evident from mixes of coral ages and mixes of molluscan faunas with
differing thermal aspects. This sequence of events took place because
glacial isostatic adjustment processes acting on North America resulted in
regional high-sea stands at ∼100 and ∼80 ka
that were higher than is the case in far-field regions, distant from large
continental ice sheets. During MIS 5e time, sea surface temperatures (SSTs)
off the Pacific coast of North America were higher than is the case at
present, evident from extralimital southern species of mollusks found in
dated deposits. Apparently, no wholesale shifts in faunal provinces took
place, but in MIS 5e time, some species of bivalves and gastropods lived
hundreds of kilometers north of their present northern limits, in good
agreement with SST estimates derived from foraminiferal records and
alkenone-based reconstructions in deep-sea cores. Because many areas of the
Pacific coast of North America have been active tectonically for much or all
of the Quaternary, many earlier interglacial periods are recorded as
uplifted, higher-elevation terraces. In addition, from southern Oregon to
northern Baja California, there are U-series-dated corals from marine
terraces that formed at ∼80 ka, during MIS 5a. In contrast to
MIS 5e, these terrace deposits host molluscan faunas that contain
extralimital northern species, indicating cooler SST at the end of MIS 5.
Here I present a review and standardized database of MIS 5e sea-level
indicators along the Pacific coast of North America and the corresponding
dated samples. The database is available in Muhs et al. (2021b; https://doi.org/10.5281/zenodo.5903285).