Geoscience Reference
In-Depth Information
of freshwater forcing, leading to an increase in salinity.
Convection in the GIN Seas is then forced by a (partial)
recovery of the AMOC and associated salinity transport.
Overall, Cheng et al. provide a convincing mechanism, in
which both local and remote salinity feedbacks play a role in
the AMOC recovery.
New data from four sediment cores in addition to the
synthesis of earlier results from the Labrador margin [Rashid
et al., this volume] shed light on the development of the YD
event. There has been a long-standing debate about the origin
of YD event [e.g., Mercer, 1969; Johnson and McClure,
1976; Ruddiman and McIntyre, 1981; Teller and Thorleifdson,
1983; Teller et al., 2005; Broecker et al., 1989; Andrews et al.,
1995; Lowell et al., 2005]. Most of these studies propose
freshwater
aines of inner Titcomb Lakes (Wind River Range, Wyoming,
United States) and Waiho Loop of New Zealand. The sam-
pling strategies prescribed may help in minimizing problems
with de
ning true age of the moraines. The chapter includes
a guide for determining the minimum number of samples
that must be collected to answer a particular paleoclimate
question.
During the termination of the last ice age, atmospheric CO 2
rapidly increased in two steps [Monnin et al., 2001], and
atmospheric
14 C atm ) decreased by ~190% between
17.5 and 14.5 ka [Reimer et al., 2009] (Figure 3), coined the
14 C(
Δ
Δ
Mystery Interval
by Denton et al. [2006]. From two eastern
Paci
c sediment cores off Baja California, Marchitto et al.
[2007] have documented two strong negative excursions of
Δ
14 C, which were corroborated by another eastern Paci
flooding at sites of deepwater formation in the
North Atlantic; however, a freshwater signature has not been
found in proxy records near these sites thus far. Rashid et al.
show that a
c
record of Stott et al. [2009, this volume] and a record from
the western Arabian Sea [Bryan et al., 2010]. One of those
negative excursions of
18 O depletion in planktonic foraminifers indi-
cates a YD freshwater signature in the Hudson Strait but not
in the more distal cores despite the presence of a H0 high-
carbonate bed in these cores. Rashid et al. hypothesize that if
the fresh water discharged through Hudson Strait was ad-
mixed with fine-grained detrital carbonates, it would form a
hyperpycnal
14 C corresponds to the Mystery
Interval and the other to the YD. The emerging understand-
ing is that during the LGM, there was a hydrographic divide
between the upper 2 km of the water column and the deep
Southern Ocean, where dense and salty deep waters hosted
the depleted Δ
δ
Δ
14 C[Adkins et al., 2002]. Accordingly, these
14 C waters were termed the
ow transported through the deep Labrador Sea
Northwest Atlantic Mid-Ocean Channel to distal sites of the
NW Atlantic Ocean such as the Sohm Abyssal Plain. With
the release of entrained sediment, fresh water from the
hyperpycnal
depleted
Δ
Mystery Reservoir.
The origin of the
has been linked to
freshwater release into the North Atlantic during H1 and YD
(H0) [Toggweiler, 2009]. It has been further inferred that the
resulting AMOC weakening initiated a chain of events reach-
ing to the tropics and Southern Hemisphere [Anderson et al.,
2009]. According to this interpretation, as the northward heat
Mystery Reservoir
flow would buoyantly rise and lose its signature
by mixing with the ambient sea water. This mechanism
explains a lack of YD
18 O depletion in sediment cores
retrieved from the NWAtlantic. Thus, the long-sought smok-
ing gun for the signature of YD (H0) freshwater flood [e.g.,
Broecker et al., 1989] remains elusive.
Applegate and Alley [this volume] evaluate the potential of
using cosmogenic radionuclides (CRN) to date glacial mo-
raine boulders to trace the geographic expression of abrupt
climate change. The chapter provides a synthesis of the
current state of the knowledge using CRN to determine the
extent and retreat of glaciers in a terrestrial setting. Problems
highlighted deal with selecting samples for exposure dating
and with the calculation of exposure dates from nuclide
concentrations. Failure to address these issues will yield
too-young exposure dates on moraines that have lost material
from their crests over time. In addition, geomorphic processes
are likely to introduce errors into the calibration of nuclide
production rates. The authors point to a conclusion from a
recent study by Vacco et al. [2009] that the ages of true YD
moraines should cluster around the end of YD, however the
recent modeling study complicated this simplistic assump-
tion. Applegate and Alley demonstrate their points using
beryllium-10 exposure dates from two
δ
flow slowed, the heat accumulated in the tropics and warmed
the Southern Ocean, resulting in the reduction of sea ice
extent around Antarctica. This sea ice retreat shifted the
Southern Hemisphere westerlies poleward through a mecha-
nism that remains unknown, allowing the ventilation of the
deep ocean that stored CO 2 and some other chemical com-
ponents such as silica. As a result, a rise in atmospheric CO 2 ,
depleted Δ
14 C atm , and a dramatic increase in the accumula-
tion of siliceous sediments were observed in the Southern
Ocean [Anderson et al., 2009].
Broecker and Barker [2007], Broecker [2009], and
Broecker and Clark [2010] launched an extensive search for
the
nd
any evidence for it. Stott and Timmermann [this volume] put
forward a provocative
Mystery Reservoir
in the Paci
c Ocean and did not
CO 2 capacitor
hypothesis that re-
sembles the
hypothesis of Kennett et al.
[2003] and offers a way to account for a decrease in the
depleted
clathrate gun
14 C atm and increase in the atmospheric CO 2 . The
main point of the
Δ
hypothesis is the pre-
sumed existence of an unspecified source of CO 2 that can
explain both the glacial to interglacial CO 2 change and the
CO 2 capacitor
not so primed
mor-
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