ALBERT

Description: An early Holocene record from the Gulf of Mexico (GOM) reveals climatic and hydrologic changes during the interval from 10.5 to 7 thousand calendar years before present from paired analyses of Mg/Ca and d18O on foraminiferal calcite. The sea surface temperature record based on foraminiferal Mg/Ca contains six oscillations and an overall ~1.5°C warming that appears to be similar to the September-March insolation difference. The d18O of seawater in the GOM (d18OGOM) record contains six oscillations, including a -0.8 per mil excursion that may be associated with the "8.2 ka climate event" or a broader climate anomaly. Faunal census records from three GOM cores exhibit similar changes, suggesting subcentennial-scale variability in the incursions of Caribbean waters into the GOM. Overall, our results provide evidence that the subtropics were characterized by decadal- to centennial-scale climatic and hydrologic variability during the early Holocene.

Description: A molecular organic geochemical proxy (TEX86) for sea surface temperature (SST) is compared with a foraminifera-based SST proxy (Mg/Ca) in a decadal-resolution marine sedimentary record spanning the last 1000 years from the Gulf of Mexico. We assess the relative strengths of the organic and inorganic paleoceanographic techniques for reconstructing high-resolution SST variability during recent climate events, including the Little Ice Age (LIA) and the Medieval Warm Period (MWP). SST estimates based on the molecular organic proxy TEX86 show a similar magnitude and pattern of SST variability to foraminiferal Mg/Ca-SST estimates but with some important differences. For instance, both proxies show a cooling (1°C-2°C) of Gulf of Mexico SSTs during the LIA. During the MWP, however, Mg/Ca-SSTs are similar to near-modern SSTs, while TEX86 indicates SSTs that were cooler than modern. Using the respective SST calibrations for each proxy results in TEX86-SST estimates that are 2°C-4°C warmer than Mg/Ca-SST throughout the 1000 year record. We interpret the TEX86-SST as a summer-weighted SST signal from the upper mixed layer, whereas the Mg/Ca-SST better reflects the mean annual SST. Downcore differences in the SST estimates between the two proxies (DeltaT = TEX86 - Mg/Ca) are interpreted in the context of varying seasonality and/or changing water column temperature gradients.

Description: High-resolution stable isotopic records are presented for the epi-benthic foraminifer Cibicidoides, the inferred shallow-dwelling planktonic Globigerinoides quadrilobatus, and the inferred deep-dwelling planktonic Globoquadrina dehiscens from the middle Miocene (~16-12 Ma) of Deep Sea Drilling Project site 588A, Lord Howe Rise, southwest Pacific. High-resolution, multiple species oxygen and carbon isotopic data define the timing and character of the well-known middle Miocene climatic-oceanographic transition with a resolution comparable to Quaternary records. The benthic foraminiferal delta18O record is marked by several large fluctuations from ~16 to 14.8 Ma, followed by a series of rapid (〈50 kyr) delta18O increases that suggest a new state of the ocean-climate system after 14.8 Ma. The total middle Miocene benthic oxygen isotopic increase of 1.2 per mil is largely incorporated in two steps, an increase of 0.8 per mil from 14.5 to 14.0 Ma and a second increase of 0.7 per mil from 13.45 to 12.45 Ma. Each step is comprised of a series of marked delta18O increases, indicative of rapid East Antarctic ice sheet growth and contemporaneous deepwater cooling. A strong covariance of 0.7 per mil between the benthic and deep-dwelling planktonic species from 14.5 to 14.0 Ma (including a rapid increase from 14.1 to 14.05 Ma) suggests a 0.7 per mil increase in the delta18O composition of seawater (delta18Osw) because of East Antarctic ice sheet growth. Comparison of the delta18O record of Gs. quadrilobatus suggests that surface waters warmed at this site by ~3°C from 14.1 to 13.6 Ma. Carbon isotopic time series for each species generally covary throughout the early to middle Miocene interval (~16-12 Ma), confirming that delta13C variations in this interval largely represent reservoir changes. High-resolution delta13C data allow improved resolution of the latter five of six delta13C maxima within the well-known early to middle Miocene carbon isotopic excursion (the Monterey Carbon Isotopic Excursion from 17.0 to 13.5 Ma). This is useful for global correlation. The last of these maxima ends with a 1 per mil decrease centered from 13.9 to 13.7 Ma, ~300 kyr after the delta18O increase considered to reflect East Antarctic ice growth. Covariance between benthic delta18O and delta13C from ~16 to 13.8 Ma suggests a sensitive relation between global carbon cycling and the ocean-climate system prior to 13.8 Ma. Episodic increases in organic carbon burial may have contributed to deep-sea benthic delta13C maxima and synchronous global cooling. The positive relationship ended at ~13.8 Ma, indicative of changing relations between global carbon cycling and the ocean-climate system brought on by the increased stability of the East Antarctic ice sheet after a major growth phase from 14.5 to 14.0 Ma.

Description: A leading hypothesis to explain abrupt climate change during the last glacial cycle calls on fluctuations in the margin of the North American Laurentide Ice Sheet (LIS), which may have routed fresh water between the Gulf of Mexico (GOM) and the North Atlantic, affecting North Atlantic Deep Water variability and regional climate. Paired measurements of d18O and Mg/Ca of foraminiferal calcite from GOM sediments reveal five episodes of LIS meltwater input from 28 to 45 thousand years ago (ka) that do not match the millennial-scale Dansgaard-Oeschger warmings recorded in Greenland ice. We suggest that summer melting of the LIS may occur during Antarctic warming and likely contributed to sea level variability during marine isotope stage 3.

Description: During the last deglaciation, Greenland ice core and North Atlantic sediment records exhibit multiple abrupt climate events including the Younger Dryas cold episode (12.9-11.7 ka). However, evidence for the presence of the Younger Dryas in the Gulf of Mexico (GOM) and the relationship between GOM sea surface temperature (SST) and high-latitude climate change is less clear. We present new Mg/Ca-SST records from two varieties of the planktonic foraminifer Globigerinoides ruber (white and pink) to assess northern GOM SST history from approximately 18.4-10.8 ka. Thirty-five accelerator mass spectrometry (AMS) 14C dates from Orca Basin core MD02-2550 provide excellent age control and document high sedimentation rates (~40 cm/kyr). G. ruber (white and pink) Mg/Ca-SST data exhibit increases (~4.6 ± 0.6°C and ~2.2 ± 0.5°C, respectively) from at least 17.8-16.6 ka, with nearly decadal resolution that are early relative to the onset of the Bolling-Allerod interstadial. Moreover, G. ruber (white) SST decreases at 16.0-14.7 ka (~1.0 ± 0.5°C) and 12.8-11.6 ka (~2.4 ± 0.6°C) correlate to the Oldest and Younger Dryas in Greenland and Cariaco Basin. The G. ruber (pink) SST record, which reflects differences in seasonality and/or depth habitat, is often not in phase with G. ruber (white) and closely resembles Antarctic air temperature records. Overall, it appears that Orca Basin SST records follow Antarctic air temperature early in the deglacial sequence and exhibit enhanced seasonality during Greenland stadials.