Variability and Trends in Physical and Biogeochemical Parameters of the Mediterranean Sea during a Cruise with RV MARIA S. MERIAN in March 2018

The last few decades have seen dramatic changes in the hydrography and biogeochemistry of the Mediterranean Sea. The complex bathymetry and highly variable spatial and temporal scales of atmospheric forcing, convective and ventilation processes contribute to generate complex and unsteady circulation patterns and significant variability in biogeochemical systems. Part of the variability of this system can be influenced by anthropogenic contributions. Consequently, it is necessary to document details and to understand trends in place to better relate the observed processes and to possibly predict the consequences of these changes. In this context we report data from an oceanographic cruise in the Mediterranean Sea on the German research vessel Maria S. Merian (MSM72) in March 2018. The main objective of the cruise was to contribute to the understanding of long-term changes and trends in physical and biogeochemical parameters, such as the anthropogenic carbon uptake and to further assess the hydrographical situation after the major climatological shifts in the eastern and western part of the basin, known as the Eastern and Western Mediterranean Transients. During the cruise, multidisciplinary measurements were conducted on a predominantly zonal section throughout the Mediterranean Sea, contributing to the Med-SHIP and GO-SHIP long-term repeat cruise section that is conducted at regular intervals in the Mediterranean Sea to observe changes and impacts on physical and biogeochemical variables. The data can be accessed at https://doi.org/10.1594/PANGAEA.905902 (Hainbucher et al., 2019), https://doi.org/10.1594/PANGAEA.913512 (Hainbucher, 2020a) https://doi.org/10.1594/PANGAEA.913608, (Hainbucher, 2020b) https://doi.org/10.1594/PANGAEA.913505, (Hainbucher, 2020c) https://doi.org/10.1594/PANGAEA.905887 (Tanhua et al., 2019) and https://doi.org/10.25921/z7en-hn85 (Tanhua et al, 2020)

The Longleaf Tree-Ring Network: Reviewing and expanding the utility of Pinus palustris Mill. Dendrochronological data

The longleaf pine (Pinus palustris Mill.) and related ecosystem is an icon of the southeastern United States (US). Once covering an estimated 37 million ha from Texas to Florida to Virginia, the near-extirpation of, and subsequent restoration efforts for, the species has been well-documented over the past ca. 100 years. Although longleaf pine is one of the longest-lived tree species in the southeastern US—with documented ages of over 400 years—its use has not been reviewed in the field of dendrochronology. In this paper, we review the utility of longleaf pine tree-ring data within the applications of four primary, topical research areas: climatology and paleoclimate reconstruction, fire history, ecology, and archeology/cultural studies. Further, we highlight knowledge gaps in these topical areas, for which we introduce the Longleaf Tree-Ring Network (LTRN). The overarching purpose of the LTRN is to coalesce partners and data to expand the scientific use of longleaf pine tree-ring data across the southeastern US. As a first example of LTRN analytics, we show that the development of seasonwood chronologies (earlywood width, latewood width, and total width) enhances the utility of longleaf pine tree-ring data, indicating the value of these seasonwood metrics for future studies. We find that at 21 sites distributed across the species’ range, latewood width chronologies outperform both their earlywood and total width counterparts in mean correlation coefficient (RBAR = 0.55, 0.46, 0.52, respectively). Strategic plans for increasing the utility of longleaf pine dendrochronology in the southeastern US include [1] saving remnant material (e.g., stumps, logs, and building construction timbers) from decay, extraction, and fire consumption to help extend tree-ring records, and [2] developing new chronologies in LTRN spatial gaps to facilitate broad-scale analyses of longleaf pine ecosystems within the context of the topical groups presented

Simulation of air-sea CO2 flux in an ocean model: understanding processes and using data assimilation

The global ocean is a net sink for atmospheric carbon dioxide (CO2) under present-day climate, as the result of a complex interplay of various drivers which vary spatially and temporally. Global ocean-biogeochemical models are tools to investigate these drivers and can be applied to assess projected changes in the ocean CO2 uptake under climate change. However, model-based estimates of the present-day ocean CO$_2$ sink show discrepancies between different models and compared to observation-based estimates, highlighting the uncertainty in the estimates. The work presented in this thesis addresses three themes regarding the discrepancies and uncertainty in the simulated CO2 flux; 1) the skin temperature effect; 2) the drivers of the CO2 flux in the Southern Ocean; and 3) the use of biogeochemical data assimilation to identify and correct model biases. The focus is on one ocean-biogeochemical model, consisting of a physical model (NEMO), coupled to a biogeochemical model (MEDUSA) and a sea ice model (CICE). NEMO-MEDUSA-CICE forms the ocean component of the UK Earth System Model (UKESM1), which contributed to the latest phase of the Coupled Model Intercomparison Project. The analysis comprises runs with the coupled UKESM1 from the ensemble of historical UKESM1 runs and ocean-only runs in which NEMO-MEDUSA-CICE is forced with observation-based atmospheric data. The skin temperature effect describes a temperature gradient, typically a decrease, in the top millimeter of the ocean. When accounted for in observation-based estimates, the skin temperature effect enhances the ocean CO2 uptake, increasing the discrepancy to model-based estimates. In global models, in contrast, the skin temperature effect is typically not included. Here, the impact of including the skin temperature in NEMO-MEDUSA-CICE on the global CO2 uptake is tested in a 35 year ocean-only simulation (1980-2014). Initially, accounting for this effect leads to a transient increase in CO2 uptake. The added CO2 uptake largely accumulates in the mixed layer, which reduces the air-sea gradient in CO2, i.e. counteracting the skin temperature effect. As a result, the initial CO2 uptake anomaly decreases to about zero within 20 years of the simulation. This dynamic response in the carbon chemistry of the biogeochemical model will likely be similar in other models, hence including the skin temperature correction in the CO2 flux calculation may affect the immediate response to a CO2 uptake anomaly but will not lead to a comparable change as for the observation-based estimates. The Southern Ocean was identified as a region of net CO2 uptake but also as a region of large uncertainty. Investigating a historical UKESM1 simulation for 1980-2014 shows biases in the representation of ocean physics and biogeochemistry, which are common to other state-of-the-art Earth System models. Using the seasonal cycle of CO2 flux in the Southern Ocean as a metric for the assessment shows an opposite timing compared to the observed timing of the seasonal cycle, which indicates an imbalance or misrepresentation of the CO2 flux drivers. Investigating ocean-only runs for the same period reveals that forcing NEMO-MEDUSA-CICE with reanalysis data greatly improves the representation of the physical ocean state compared to the coupled UKESM1 run. In particular, the improvements in the physics lead to improvements in the CO2 flux representation, including a correction of the timing of the seasonal cycle. The key drivers are the mixed layer depth and summer primary production in the Sub-Antarctic. Further improvements in the CO2 flux can be achieved by initialising an ocean-only run from observation-based estimates, thereby correcting biases in the biogeochemistry. The ocean-only run identified in the Southern Ocean analysis as agreeing best with observation-based estimates is taken forward to investigate potential further improvements in the CO2 flux and variability using biogeochemical data assimilation. Data assimilation is a statistical method to update model fields or parameters with observations. A 3D-variational method (NEMOVAR) is updated and tuned for this purpose, i.e. to optimally assimilate data into the selected ocean-only run from a range of data sources (satellite, ship, BGC-Argo floats). Previous capabilities of biogeochemical data assimilation with NEMOVAR, which were limited to satellite and synthetic in situ data, are extended towards new variables and actual observations. Experiments assimilating the data sources individually and in combination reveal the effects the characteristics of different observation sources (surface or profiles, coverage, etc.) have on the analysis, with BGC-Argo assimilation yielding coherent impacts on the biogeochemistry. The assimilation highlights regional misrepresentation of the biogeochemistry, confirming the findings from the previous Southern Ocean investigation. The assimilation is also capable of regionally improving the CO2 flux variability compared to the non-assimilative run. The results suggest various potential routes to explore for further improvements of the assimilation system, to address the main limitations of observational data coverage, the assimilation method, and exploring questions beyond the CO2 flux in the Southern Ocean. Ultimately, these and future efforts in data assimilation should yield a robust framework to combine models and observations, exploiting their individual benefits, to improve our understanding of and reducing uncertainty in the ocean CO2 uptake.Natural Environment Research Council (NERC

Dendrochronological investigation of the Knob Creek Cabin at President Abraham Lincoln\u27s boyhood home, Hodgenville, Kentucky, USA

The Boyhood Home at Knob Creek is part of the Abraham Lincoln Birthplace National Historical Park in Hodgenville, Kentucky. Here, a replica cabin constructed in the 1930s by Chester and Hattie Howard is meant to represent the home where President Abraham Lincoln formed his earliest memories. According to oral histories, this replica, the Knob Creek Cabin, was constructed using timbers salvaged from a cabin found elsewhere on the Knob Creek Farm, believed to have been the home of Lincoln\u27s childhood friend, Austin Gollaher. Therefore, the cabin is believed to be contemporary with Lincoln\u27s childhood at Knob Creek (early 1800s), and represents what the original Lincoln family cabin may have looked like, despite the disappearance of the original Lincoln cabin during the late 1800s. To determine whether the Knob Creek Cabin truly dates to Lincoln\u27s childhood, we extracted core samples from logs in the cabin and compared the tree-ring patterns to an absolutely-dated reference chronology from Mammoth Cave, Kentucky. We dated the oak chronology from the Knob Creek Cabin to the period 1712–1861 (r = 0.57, n = 150 years, t = 8.44, p \u3c 0.0001). Several of the logs indicate cutting dates occurred during two different periods, 1847–1848 and 1861–1863, neither of which coincide with Lincoln\u27s childhood at Knob Creek Farm. Based on these findings, we conclude that the Knob Creek Cabin could not have been originally constructed by the Gollaher family during Abraham Lincoln\u27s childhood on the Knob Creek Farm. Instead, the Knob Creek Cabin likely was built using timbers that were salvaged from two different structures, one that was constructed during the period 1847–1848 and another that was constructed during the period 1861–1863. The origins of these logs are unknown

Using Dendroecology to Strengthen the Historic Integrity of Cumberland Homesteads Tower in Crossville, Tennessee

The Cumberland Homesteads Historic District, located on the Cumberland Plateau in East Tennessee, is home to one of the first and largest Homesteads projects attempted during the New Deal era. Although the settlement did not succeed in its original objective, the history of the Cumberland Homesteads has become a valued foundation of the local community, which in turn strives to protect the legacy of the Cumberland Homesteads Tower. To preserve the integrity of the structure as well as the historical integrity of the landscape, the Cumberland Homesteads Tower Association sought to date and potentially remove trees that were not present during the period of significance (prior to 1938). The majority of the trees in close proximity to the Tower were identified as Eastern hemlock (Tsuga canadensis (L.) Carrière) and 15 trees total were sampled. Additionally, three post oak (Quercus stellata Wangenh.) trees located in a historic ‘triangle’ across the highway from the Tower and targeted for removal were sampled. Samples were successfully dated, and ca. half of the hemlock were confirmed to have been planted after the construction of the Homesteads Tower. Additionally, post oaks analyzed near the Tower were dated back to the early 1800s, which motivated their protection in the midst of a road project threatening their survival

Tree-Ring Based Reconstruction of Historical Fire in an Endangered Ecosystem in the Florida Keys

Big Pine Key, Florida, is home to one of Earth’s largest swaths of the critically-endangered dry forests. Known as pine rocklands, this fire-adapted ecosystem must experience regular fire to persist and remain healthy. Pine rocklands are composed of a sole canopy species: the South Florida slash pine (Pinus elliottii var. densa), along with a dense understory of various woody and herbaceous species, and minimal surface moisture and soil development. Slash pine record wildfire activity of the surrounding area via fire scars preserved within the annual tree rings formed by the species. Our study used dendrochronology to investigate the fire history of the pine rocklands on Big Pine Key, specifically within and around the National Key Deer Refuge (NKDR) because it is the largest segment of unfragmented pine rockland on the island. We combined the results found within the NKDR with those of a previous study completed in 2011, and incorporated historical documents and reports of prescribed and natural fires through November 2019 into our evaluation of fire history on Big Pine Key. We conclude that prescribed burning practices are vital to truly restore natural fire behavior, and repeated burning on these islands in the future must be prioritized

Tree-Ring Based Reconstruction of Historical Fire in an Endangered Ecosystem in the Florida Keys

Big Pine Key, Florida, is home to one of Earth’s largest swaths of the critically-endangered dry forests. Known as pine rocklands, this fire-adapted ecosystem must experience regular fire to persist and remain healthy. Pine rocklands are composed of a sole canopy species: the South Florida slash pine (Pinus elliottii var. densa), along with a dense understory of various woody and herbaceous species, and minimal surface moisture and soil development. Slash pine record wildfire activity of the surrounding area via fire scars preserved within the annual tree rings formed by the species. Our study used dendrochronology to investigate the fire history of the pine rocklands on Big Pine Key, specifically within and around the National Key Deer Refuge (NKDR) because it is the largest segment of unfragmented pine rockland on the island. We combined the results found within the NKDR with those of a previous study completed in 2011, and incorporated historical documents and reports of prescribed and natural fires through November 2019 into our evaluation of fire history on Big Pine Key. We conclude that prescribed burning practices are vital to truly restore natural fire behavior, and repeated burning on these islands in the future must be prioritized

Summer temperature variability since 1730 CE across the low-to-mid latitudes of western North America from a tree ring blue intensity network

This project was supported by the National Science Foundation under BCS- 2012482, BCS-1759694, and AGS-2002524, the United States Forest Service, the University of Idaho, and Indiana University Institute for Advanced Studies.Regional reconstructions of air temperature over the past millennium provide critical context for ongoing climate change, but they are temporally limited in the recent period or absent for many parts of the world. We demonstrate the use of latewood blue intensity (LWB) to reconstruct current-year growing (warm) season maximum temperatures (Tmax) in the low-to-mid latitudes (30°-50°N) of western North America. We present a new tree ring network comprised of 26 LWB chronologies developed from living, high-elevation Engelmann spruce (Picea engelmannii Parry ex Engelm.) sampled across the western United States. The LWB parameter shows strong, positive (r = 0.65–0.73), and temporally-stable correlations with growing season Tmax. From this network we present 4 regional Tmax reconstructions, which characterize regional temperature histories across western North America from northern Mexico to southern British Columbia over the past 4 centuries. Our comparison of these 4 temperature reconstructions highlights the spatial patterns of regional temperature trends throughout time. These reconstructions provide important updates and increased data point density to the tree ring temperature proxy network of the Northern Hemisphere. We highlight the use of blue intensity methods at both low- and mid-latitude upper tree line locations to increase the presence of strongly temperature-sensitive records at increasingly lower latitudes of the Northern Hemisphere.PostprintPeer reviewe