21 research outputs found
A new Holocene record of geomagnetic secular variation from Windermere, UK
Paleomagnetic
secular
variation
(PSV)
records
serve
as
valuable
independent
stratigraphic
correlation
and
dating
tools
for
marine
and
terrestrial
sediment
sequences,
and
enhance
knowledge
of
geomagnetic
field
dynamics.
We
present
a
new
radiocarbon-dated
record
(WINPSV-12K)
of
Holocene
geomagnetic
secular
variation
from
Windermere,
updating
the
existing
1981
UK
master
PSV
curve.
Our
analyses
used
continuous
U-channel
samples
taken
from
the
center
of
four
sediment
cores
retrieved
from
Windermere
in
2012.
The
natural
remanent
magnetization
(NRM)
of
each
U-channel
was
measured
before
and
after
stepwise
alternating
field
(AF)
demagnetization
on
a
superconducting
rock
magnetometer
at
intervals
of
0.5-cm
or
1-cm.
The
NRM
data
reveal
a
stable
and
well-defined
primary
magnetization.
Component
declinations
and
inclinations
estimated
using
Principal
Component
Analysis
(PCA)
of
NRM
data
from
the
four
Windermere
cores
correlate
well
on
their
independent
radiocarbon
age
models.
The
four
records
were
stacked
using
a
sliding
window
bootstrap
method,
resulting
in
a
composite
Holocene
PSV
record
(WINPSV-12K).
On
millennial
timescales
WINPSV-12K
correlates
well
with
other
records
from
Western
Europe
and
the
northern
North
Atlantic
to
a
resolution
of
∼
1
kyr,
given
age
uncertainties
and
spatial
variability
between
records.
WINPSV-12K
also
compares
well
to
the
CALS10k.2
and
pfm9k.1a
model
predictions
for
Windermere.
Key
regionally-significant
PSV
inclination
features
of
WINPSV-12K
which
correlate
with
other
North
Atlantic
records
include
peaks
at
5–6,
8.5,
and
10 cal ka BP,
and
a
trough
at
7calkaBP.
Key
PSV
declination
features
include
the
eastward
swing
from
5.5–2.3 cal ka BP
followed
by
a
major
westward
excursion
at
2.3 cal ka BP,
peaks
at
1.1
and
7calkaBP,
and
troughs
at
5.4
and
8.2 cal ka BP,
with
the
caveat
that
an
estimated
magnetic
lock-in
delay
of
at
least
100–200 yr
is
present.
PSV
variations
on
1–3 kyr
timescales
are
interpreted
to
represent
strengthening
and
weakening
of
the
North
American
versus
the
Siberian
and
European–Mediterranean
high-latitude
flux
lobes,
based
on
the
close
similarities
between
the
North
Atlantic
regional
records
and
the
antiphase
existing
in
the
East
Asian
Stack
record
and
the
North
East
Pacific
inclination
stack.
WINPSV-12K
provides
a
regionally-important
new
PSV
reference
curve
whose
prominent
features
may
serve
as
stratigraphic
markers
for
North
Atlantic
paleo-records
A new varve sequence from Windermere, UK, records rapid ice retreat prior to the Lateglacial Interstadial (GI-1)
Annually laminated sediments (varves) provide excellent temporal resolution to study rapid environmental change, but are rare in the early part of the Last Termination (∼19–∼11.7 ka BP). We present a new >400 varve year (vyr) varve sequence in two floating parts from Windermere, a lake at the southern margin of the mountains of northwest England. This sequence records the final retreat of the Windermere glacier at the southern edge of the Lake District Ice Cap during the transition from Heinrich Stadial 1 (∼18–∼14.7 ka BP) into the Lateglacial Interstadial (∼14.7–∼12.9 ka BP).
Laminated sediments from four lake cores from Windermere's northern and southern basins were investigated and shown to be varved. These sequences are integrated with seismic reflection evidence to reconstruct south-to-north deglaciation. Seismic and sedimentological evidence is consistent with gradual stepped ice retreat along the entire southern basin and into the northern basin between 255 and 700 vyr prior to the appearance of significant biota in the sediment that heralded the Lateglacial Interstadial, and had retreated past a recessional moraine (RM8) in the northern basin by 121 vyr prior to the interstadial. The Lateglacial Interstadial age of this biota-bearing unit was confirmed by 14C-dating, including one date from the northernmost core of ∼13.5 cal ka BP. A change in mineralogy in all four cores as the glacier retreated north of the Dent Group (the northernmost source of calcareous bedrock) and a decrease in coarse grains in the varves shows that the ice had retreated along the entire North Basin at ∼70 vyr prior to the Lateglacial Interstadial. The estimated retreat rate is 70–114 m yr−1 although buried De Geer moraines, if annual, may indicate retreat of 120 m yr−1 with a ≥3 year stillstand at a recessional moraine halfway along the basin. The glacier then retreated north of the lake basin, becoming land-terminating and retreating at 92.5–49 m yr−1.
The northernmost core has a varve sequence ending at least 111 vyr after the other core chronologies, due to the increased proximity to remnant ice in the catchment uplands into the early Lateglacial Interstadial. We show that almost all of the glacier retreat in the Windermere catchment occurred before the abrupt warming at the onset of the Lateglacial Interstadial, in keeping with similar findings from around the Irish Sea Basin, and suggesting a similar retreat timescale for other radial valley glaciers of the Lake District Ice Cap. The seismic and core evidence also show the potential for a much longer varve chronology extending at least 400 and potentially over 1000 vyr further back into Heinrich Stadial 1 (18–14.7 ka BP), suggesting that glacier retreat in the Windermere valley initiated at least before 15.5 ka BP and perhaps 16 ka BP
Palaeoseismology from microfabric and geochemical analysis of lacustrine sediment, Windermere, UK
Lake sediments commonly contain detrital layers that record events such as floods or earthquakes but these may be disturbed or partially destroyed by bioturbation. Here we use a novel combination of techniques to relate microscopic sediment fabric features to lake-basin scale processes. X-radiography and micro-XRF of cores are complemented by backscattered electron imagery and energy dispersive X-ray microanalysis of resin-embedded sediment. Together, the microfabric and geochemical methods enable the identification of clay-layer mass transport deposits despite bioturbational mixing of the original end members. Two cores with robust radionuclide chronologies contain correlative clay layers dated to 1979 (1974–1982) and 1979–1980 (1973–1986) respectively. These clay layers likely represent the distal turbidite generated by a major mass flow deposit identified from multibeam swath bathymetry and sediment grab sampling. A likely trigger for the mass flow and associated turbidity current is the 4.7 ML 1979 Carlisle earthquake. The lake basin slope was likely preconditioned for failure by increased sedimentary biogenic gas production and sediment in-wash as a result of anthropogenic activities, coupled with sediment disruption and dredging. This study highlights the effectiveness of microstratigraphic techniques in the recognition and characterisation of event layers in sediments where bioturbative disruption has occurred
Microfabric study of diatomaceous and lithogenic deposition in laminated sediments from the Gotland Deep, Baltic Sea
Large expert-curated database for benchmarking document similarity detection in biomedical literature search
Document recommendation systems for locating relevant literature have mostly relied on methods developed a decade ago. This is largely due to the lack of a large offline gold-standard benchmark of relevant documents that cover a variety of research fields such that newly developed literature search techniques can be compared, improved and translated into practice. To overcome this bottleneck, we have established the RElevant LIterature SearcH consortium consisting of more than 1500 scientists from 84 countries, who have collectively annotated the relevance of over 180 000 PubMed-listed articles with regard to their respective seed (input) article/s. The majority of annotations were contributed by highly experienced, original authors of the seed articles. The collected data cover 76% of all unique PubMed Medical Subject Headings descriptors. No systematic biases were observed across different experience levels, research fields or time spent on annotations. More importantly, annotations of the same document pairs contributed by different scientists were highly concordant. We further show that the three representative baseline methods used to generate recommended articles for evaluation (Okapi Best Matching 25, Term Frequency-Inverse Document Frequency and PubMed Related Articles) had similar overall performances. Additionally, we found that these methods each tend to produce distinct collections of recommended articles, suggesting that a hybrid method may be required to completely capture all relevant articles. The established database server located at https://relishdb.ict.griffith.edu.au is freely available for the downloading of annotation data and the blind testing of new methods. We expect that this benchmark will be useful for stimulating the development of new powerful techniques for title and title/abstract-based search engines for relevant articles in biomedical research.Peer reviewe
Tropical ocean-atmosphere controls on inter-annual climate variability in the Cretaceous Arctic
The first annually resolved sedimentary record from the Cretaceous is used to develop time series of inter-annual and decadal scale climate variability from the Arctic Ocean. Analysis of records spanning 1000 years reveals strong periodicities in the quasi-biennial oscillation and El Niño – Southern Oscillation (ENSO) band as well as a 14 year period, which all closely match periodicities typical of modern high latitude climate variability. This supports the view that an Arctic Ocean free of permanent sea ice would be driven by similar forcing to the present state, implicating tropical ocean atmosphere interaction and demonstrating that stratosphere- troposphere coupling likely played a prominent role in the transmission of Cretaceous equatorial climate forcing to polar latitudes as has recently been established for the modern earth system. On the other hand, the prominent ENSO periodicities in our records argue against the hypothesized link between past warm climates and “permanent El Niño” states
Microfabric analysis of Mn-carbonate laminae deposition and Mn-sulfide formation in the Gotland Deep, Baltic Sea
The manganese carbonate deposits of the anoxic Littorina sediments of the Gotland Deep have been commonly related to the periodic renewal of deep water by inflowing saline water from the North Sea. The use of scanning electron microscopy-based techniques allows identification of small-scale sedimentary and geochemical features associated with Mn-carbonate laminae, which has significant implications for models of Mn-carbonate formation. Varves occurring in the Littorina sequence contain up to four laminae that may be placed in a seasonal cycle, and kutnahorite laminae occur within varves only as a winter-early spring deposit. This kutnahorite laminae seasonality is in agreement with the seasonal distribution of major Baltic inflow events recorded in historical records, and a direct causal link between inflows and kutnahorite deposition is implied. Benthic foraminifera tests are found to be heavily encrusted in kutnahorite, implying that benthic recolonization during oxidation events occurs concurrently with kutnahorite formation. The relatively common occurrence of small (50 to 100 ?m) hexagonal ?-Mn-sulfide pseudomorphs, associated with 13% of kutnahorite laminae studied, is reported in Gotland Deep sediments for the first time. Although Mn-sulfide crystals are not usually preserved in the sediment, the discovery of Mn-sulfide pseudomorphs suggests that initial formation of Mn-sulfide in the Gotland Deep may occur much more commonly during the process of kutnahorite formation than previous reports of Mn-sulfide occurrence have implied
High diatom production and export in stratified waters – A potential negative feedback to global warming
It is widely held that increased stratification and reduced vertical mixing in the ocean driven by global warming will promote the replacement of diatoms by smaller phytoplankton and lead to an overall decrease in productivity and carbon export. Here we present contrary evidence from a synergy of modern observations and palaeo-records that reveal high diatom production and export from stratified waters. Diatom adaptations to stratified waters include the ability to grow in low light conditions in deep chlorophyll maxima; vertical migrations between nutricline depths and the surface, and symbioses with N2-fixing cyanobacteria in diatom–diazotroph associations (DDA). These strategies foster the maintenance of seed populations that may then exploit mixing events induced by storms or eddies, but may also inherently promote blooms. Recent oceanographic observations in the subtropical gyres, at increasingly high temporal and spatial resolutions, have monitored short-lived but often substantial blooms and export of stratified-adapted diatoms including rhizosolenids and the diazotroph-associated Hemiaulus hauckii. Aggregate formation by such diatoms is common and promotes rapid settling thereby minimizing water column remineralization and optimizing carbon flux. Convergence zones associated with oceanic fronts or mesoscale features may also generate substantial flux of stratified-adapted diatom species. Conventional oceanographic observing strategies and sampling techniques under-represent such activity due to the lack of adequate capability to sample the large sized diatoms and colonies involved, the subsurface location of many of these blooms, their common development in thin <3 m layers and their episodic nature. Superbly preserved “palaeo-sediment trap” records in laminated sediments reveal enhanced production and export from stratified waters mediated by the same taxa. The organic carbon-rich Mediterranean sapropels were the result of production within deep chlorophyll maxima in highly stratified waters dominated by rhizosolenid diatoms but also including H. hauckii. In the Cretaceous when CO2 levels were higher than present, laminated sediments reveal dominant flux also including Hemiaulus and rhizosolenid species. Nitrogen isotope and biomarker studies suggest that, analogous to modern DDA blooms within the subtropical gyres, Hemiaulus blooms in the ancient Mediterranean and within the Cretaceous seas were aided by nitrogen-fixing cyanobacterial symbionts. These lines of evidence suggest that diatom production and associated export of organic carbon, may not decrease, as is widely predicted, but may actually increase with greater ocean stratification, and potentially act as a negative feedback to global warming. However, the key genera involved in such potential feedbacks are underrepresented in both laboratory and field studies and are poorly represented in models. Our findings suggest that a reappraisal is necessary of the way diatoms are represented as plankton functional types (PFTs) in ocean biogeochemical models and that new observing and sampling strategies are also required to study these processes
The case of the diatoms and the muddled mandalas: Time to recognize diatom adaptations to stratified waters
Models used to predict future ocean ecosystem and biogeochemical behaviour depend on simplified ecological frameworks allowing the definition of plankton functional types. Foremost among such frameworks has been the mandala of Margalef. His 1978 paper has been increasingly referred to in the past decade as simplified ecological schemes have been sought to help predict the effect of climate change on phytoplankton. However, the mandala is based on an understanding of the subject that is over 40 years old, when observational studies were largely limited to the coastal ocean and to near surface waters. Furthermore, most recent reproductions of the mandala are significant oversimplifications of Margalef’s original. In these simplified mandala-type constructs, diatoms, in particular, have commonly been cast as a single plankton functional type that thrive in turbulent waters and decrease in abundance with increasing stratification. On this basis, it is widely predicted that diatom productivity and hence the effectiveness of the marine biological carbon pump will decrease with climate change that is driving increased stratification of the oceans. But Margalef’s original took a more refined approach and depicted diatom genera that were adapted to more stratified conditions such as those characteristic of the subtropical oligotrophic gyres. If we now draw on the vast advances in observational oceanography of recent decades it is evident that diatoms may thrive, bloom and generate significant export even in the most intensely stratified and apparently oligotrophic conditions. Indeed, some diatom species have unique adaptations to such environments. We therefore suggest that it is time to abandon oversimplified schemes and recognize the diverse ecology of diatoms